fix: Use `png_voidp` instead of `voidp` in pngread.c and pngwrite.c

A benign type error introduced in libpng-1.0.9beta1 (Nov 2000) went
unnoticed in reviews and testing for nearly 25 years.

The type `voidp` (from zconf.h via zlib.h) is a plain "void pointer".
By contrast, `png_voidp` (from pngconf.h via png.h) was defined as a
"void FAR pointer" on legacy i8086 memory models (MSDOS, OS/2 2.x,
Win16), and as a plain "void pointer" elsewhere. The support for FAR
pointers in mixed-memory models was removed in libpng-1.6.0, which
inadvertently "fixed" the underlying error. However, all libpng
versions from 1.0.9 to 1.5.30 (Nov 2000–Sep 2017) remained broken on
these models; and yet, nobody noticed.

This typo illustrates how long we've been carrying pointer abstraction
baggage without any confirmed benefit.

This is a cherry-pick of commit 0ff384943db3732fe6fe9e132b00eb3373072b41
from branch 'libpng18'.

Reviewed-by: Chris Blume <ProgramMax@gmail.com>
Reviewed-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>

pngread.c

diff --git a/pngread.c b/pngread.c
index 13a93de..7967cc2 100644
--- a/pngread.c
+++ b/pngread.c
@@ -867,7 +867,7 @@
 #ifdef PNG_INFO_IMAGE_SUPPORTED
 void PNGAPI
 png_read_png(png_structrp png_ptr, png_inforp info_ptr,
-    int transforms, voidp params)
+    int transforms, png_voidp params)
 {
    png_debug(1, "in png_read_png");
 

pngwrite.c

diff --git a/pngwrite.c b/pngwrite.c
index 8314896..924b1e0 100644
--- a/pngwrite.c
+++ b/pngwrite.c
@@ -1393,7 +1393,7 @@
 #ifdef PNG_INFO_IMAGE_SUPPORTED
 void PNGAPI
 png_write_png(png_structrp png_ptr, png_inforp info_ptr,
-    int transforms, voidp params)
+    int transforms, png_voidp params)
 {
    png_debug(1, "in png_write_png");