[libpng16] Improved pngstest speed by not doing redundant tests and add const

to the background parameter of png_image_finish_read. The --background
option is now done automagically only when required, so that commandline
option no longer exists.

pngread.c

diff --git a/pngread.c b/pngread.c
index 7ff4d08..c4cc05a 100644
--- a/pngread.c
+++ b/pngread.c
@@ -1191,7 +1191,7 @@
    png_voidp  buffer;
    png_int_32 row_stride;
    png_voidp  colormap;
-   png_colorp background;
+   png_const_colorp background;
    /* Local variables: */
    png_bytep       local_row;
    png_bytep       first_row;
@@ -3882,8 +3882,8 @@
 }
 
 int PNGAPI
-png_image_finish_read(png_imagep image, png_colorp background, void *buffer,
-   png_int_32 row_stride, void *colormap)
+png_image_finish_read(png_imagep image, png_const_colorp background,
+   void *buffer, png_int_32 row_stride, void *colormap)
 {
    if (image != NULL && image->version == PNG_IMAGE_VERSION)
    {