[libpng16] Implemented 'restrict' for png_info and png_struct.

Added new "png_structrp" typedef. Because of the
way libpng works both png_info and png_struct are always accessed via a
single pointer.  This means adding C99 'restrict' to the pointer gives
the compiler some opportunity to optimize the code.  This change allows that.

pngerror.c

diff --git a/pngerror.c b/pngerror.c
index 8e28e9e..c2b8c73 100644
--- a/pngerror.c
+++ b/pngerror.c
@@ -20,12 +20,12 @@
 
 #if defined(PNG_READ_SUPPORTED) || defined(PNG_WRITE_SUPPORTED)
 
-static PNG_FUNCTION(void, png_default_error,PNGARG((png_const_structp png_ptr,
+static PNG_FUNCTION(void, png_default_error,PNGARG((png_const_structrp png_ptr,
     png_const_charp error_message)),PNG_NORETURN);
 
 #ifdef PNG_WARNINGS_SUPPORTED
 static void /* PRIVATE */
-png_default_warning PNGARG((png_const_structp png_ptr,
+png_default_warning PNGARG((png_const_structrp png_ptr,
    png_const_charp warning_message));
 #endif /* PNG_WARNINGS_SUPPORTED */
 
@@ -36,7 +36,7 @@
  */
 #ifdef PNG_ERROR_TEXT_SUPPORTED
 PNG_FUNCTION(void,PNGAPI
-png_error,(png_const_structp png_ptr, png_const_charp error_message),
+png_error,(png_const_structrp png_ptr, png_const_charp error_message),
    PNG_NORETURN)
 {
 #ifdef PNG_ERROR_NUMBERS_SUPPORTED
@@ -80,7 +80,7 @@
    }
 #endif
    if (png_ptr != NULL && png_ptr->error_fn != NULL)
-      (*(png_ptr->error_fn))(png_constcast(png_structp,png_ptr), error_message);
+      (*(png_ptr->error_fn))(png_constcast(png_structrp,png_ptr), error_message);
 
    /* If the custom handler doesn't exist, or if it returns,
       use the default handler, which will not return. */
@@ -88,7 +88,7 @@
 }
 #else
 PNG_FUNCTION(void,PNGAPI
-png_err,(png_const_structp png_ptr),PNG_NORETURN)
+png_err,(png_const_structrp png_ptr),PNG_NORETURN)
 {
    /* Prior to 1.5.2 the error_fn received a NULL pointer, expressed
     * erroneously as '\0', instead of the empty string "".  This was
@@ -212,7 +212,7 @@
  * png_set_error_fn() to replace the warning function at run-time.
  */
 void PNGAPI
-png_warning(png_const_structp png_ptr, png_const_charp warning_message)
+png_warning(png_const_structrp png_ptr, png_const_charp warning_message)
 {
    int offset = 0;
    if (png_ptr != NULL)
@@ -231,7 +231,7 @@
       }
    }
    if (png_ptr != NULL && png_ptr->warning_fn != NULL)
-      (*(png_ptr->warning_fn))(png_constcast(png_structp,png_ptr),
+      (*(png_ptr->warning_fn))(png_constcast(png_structrp,png_ptr),
          warning_message + offset);
    else
       png_default_warning(png_ptr, warning_message + offset);
@@ -280,7 +280,7 @@
 }
 
 void
-png_formatted_warning(png_const_structp png_ptr, png_warning_parameters p,
+png_formatted_warning(png_const_structrp png_ptr, png_warning_parameters p,
    png_const_charp message)
 {
    /* The internal buffer is just 128 bytes - enough for all our messages,
@@ -349,7 +349,7 @@
 
 #ifdef PNG_BENIGN_ERRORS_SUPPORTED
 void PNGAPI
-png_benign_error(png_const_structp png_ptr, png_const_charp error_message)
+png_benign_error(png_const_structrp png_ptr, png_const_charp error_message)
 {
   if (png_ptr->flags & PNG_FLAG_BENIGN_ERRORS_WARN)
      png_warning(png_ptr, error_message);
@@ -373,7 +373,7 @@
 #define PNG_MAX_ERROR_TEXT 64
 #if defined(PNG_WARNINGS_SUPPORTED) || defined(PNG_ERROR_TEXT_SUPPORTED)
 static void /* PRIVATE */
-png_format_buffer(png_const_structp png_ptr, png_charp buffer, png_const_charp
+png_format_buffer(png_const_structrp png_ptr, png_charp buffer, png_const_charp
     error_message)
 {
    png_uint_32 chunk_name = png_ptr->chunk_name;
@@ -419,7 +419,7 @@
 
 #if defined(PNG_READ_SUPPORTED) && defined(PNG_ERROR_TEXT_SUPPORTED)
 PNG_FUNCTION(void,PNGAPI
-png_chunk_error,(png_const_structp png_ptr, png_const_charp error_message),
+png_chunk_error,(png_const_structrp png_ptr, png_const_charp error_message),
    PNG_NORETURN)
 {
    char msg[18+PNG_MAX_ERROR_TEXT];
@@ -436,7 +436,7 @@
 
 #ifdef PNG_WARNINGS_SUPPORTED
 void PNGAPI
-png_chunk_warning(png_const_structp png_ptr, png_const_charp warning_message)
+png_chunk_warning(png_const_structrp png_ptr, png_const_charp warning_message)
 {
    char msg[18+PNG_MAX_ERROR_TEXT];
    if (png_ptr == NULL)
@@ -453,7 +453,7 @@
 #ifdef PNG_READ_SUPPORTED
 #ifdef PNG_BENIGN_ERRORS_SUPPORTED
 void PNGAPI
-png_chunk_benign_error(png_const_structp png_ptr, png_const_charp error_message)
+png_chunk_benign_error(png_const_structrp png_ptr, png_const_charp error_message)
 {
    if (png_ptr->flags & PNG_FLAG_BENIGN_ERRORS_WARN)
       png_chunk_warning(png_ptr, error_message);
@@ -467,7 +467,7 @@
 #ifdef PNG_ERROR_TEXT_SUPPORTED
 #ifdef PNG_FLOATING_POINT_SUPPORTED
 PNG_FUNCTION(void,
-png_fixed_error,(png_const_structp png_ptr, png_const_charp name),PNG_NORETURN)
+png_fixed_error,(png_const_structrp png_ptr, png_const_charp name),PNG_NORETURN)
 {
 #  define fixed_message "fixed point overflow in "
 #  define fixed_message_ln ((sizeof fixed_message)-1)
@@ -491,7 +491,7 @@
  * otherwise it is necessary for png_default_error to be overridden.
  */
 jmp_buf* PNGAPI
-png_set_longjmp_fn(png_structp png_ptr, png_longjmp_ptr longjmp_fn,
+png_set_longjmp_fn(png_structrp png_ptr, png_longjmp_ptr longjmp_fn,
     size_t jmp_buf_size)
 {
    /* From libpng 1.6.0 the app gets one chance to set a 'jmpbuf_size' value
@@ -560,7 +560,7 @@
 }
 
 void /* PRIVATE */
-png_free_jmpbuf(png_structp png_ptr)
+png_free_jmpbuf(png_structrp png_ptr)
 {
    if (png_ptr != NULL)
    {
@@ -605,7 +605,7 @@
  * error function pointer in png_set_error_fn().
  */
 static PNG_FUNCTION(void /* PRIVATE */,
-png_default_error,(png_const_structp png_ptr, png_const_charp error_message),
+png_default_error,(png_const_structrp png_ptr, png_const_charp error_message),
    PNG_NORETURN)
 {
 #ifdef PNG_CONSOLE_IO_SUPPORTED
@@ -652,7 +652,7 @@
 }
 
 PNG_FUNCTION(void,PNGAPI
-png_longjmp,(png_const_structp png_ptr, int val),PNG_NORETURN)
+png_longjmp,(png_const_structrp png_ptr, int val),PNG_NORETURN)
 {
 #ifdef PNG_SETJMP_SUPPORTED
    if (png_ptr && png_ptr->longjmp_fn && png_ptr->jmp_buf_ptr)
@@ -670,7 +670,7 @@
  * not used, but it is passed in case it may be useful.
  */
 static void /* PRIVATE */
-png_default_warning(png_const_structp png_ptr, png_const_charp warning_message)
+png_default_warning(png_const_structrp png_ptr, png_const_charp warning_message)
 {
 #ifdef PNG_CONSOLE_IO_SUPPORTED
 #  ifdef PNG_ERROR_NUMBERS_SUPPORTED
@@ -720,7 +720,7 @@
  * method used in the default routine calls longjmp(png_ptr->jmp_buf_ptr, 1)
  */
 void PNGAPI
-png_set_error_fn(png_structp png_ptr, png_voidp error_ptr,
+png_set_error_fn(png_structrp png_ptr, png_voidp error_ptr,
     png_error_ptr error_fn, png_error_ptr warning_fn)
 {
    if (png_ptr == NULL)
@@ -741,7 +741,7 @@
  * pointer before png_write_destroy and png_read_destroy are called.
  */
 png_voidp PNGAPI
-png_get_error_ptr(png_const_structp png_ptr)
+png_get_error_ptr(png_const_structrp png_ptr)
 {
    if (png_ptr == NULL)
       return NULL;
@@ -752,7 +752,7 @@
 
 #ifdef PNG_ERROR_NUMBERS_SUPPORTED
 void PNGAPI
-png_set_strip_error_numbers(png_structp png_ptr, png_uint_32 strip_mode)
+png_set_strip_error_numbers(png_structrp png_ptr, png_uint_32 strip_mode)
 {
    if (png_ptr != NULL)
    {
@@ -773,7 +773,7 @@
 png_safe_error,(png_structp png_nonconst_ptr, png_const_charp error_message),
    PNG_NORETURN)
 {
-   const png_const_structp png_ptr = png_nonconst_ptr;
+   const png_const_structrp png_ptr = png_nonconst_ptr;
    png_imagep image = png_voidcast(png_imagep, png_ptr->error_ptr);
 
    /* An error is always logged here, overwriting anything (typically a warning)
@@ -807,7 +807,7 @@
 void /* PRIVATE */
 png_safe_warning(png_structp png_nonconst_ptr, png_const_charp warning_message)
 {
-   const png_const_structp png_ptr = png_nonconst_ptr;
+   const png_const_structrp png_ptr = png_nonconst_ptr;
    png_imagep image = png_voidcast(png_imagep, png_ptr->error_ptr);
 
    /* A warning is only logged if there is no prior warning or error. */