"test-png" tool to find PNGs that cause warnings

New tool, "test-png", which exercises libpng library on
a set of PNG image files, allowing us to find those which
cause Tux Paint (via SDL_image, in turn via libpng) to
report back warnings to STDERR.

h/t Tim Dickson, per https://sourceforge.net/p/tuxpaint/bugs/252/

.gitignore

@@ -16,3 +16,4 @@ TuxPaint.dmg
 src/po/POTFILES.in
 src/tuxpaint.desktop
 src/org.tuxpaint.Tuxpaint.appdata.xml
+test-png

Makefile

@@ -4,7 +4,7 @@
 # Various contributors (see AUTHORS.txt)
 # http://www.tuxpaint.org/
 
-# June 14, 2002 - June 29, 2022
+# June 14, 2002 - July 3, 2022
 
 
 # The version number, for release:
@@ -315,6 +315,9 @@ NOSVGFLAG:=$(if $(SVG_LIB),,-DNOSVG$(warning No SVG for you!))
 OLDSVGFLAG:=$(if $(filter -lsvg-cairo,$(SVG_LIB)),-DOLD_SVG,)
 
 
+PNG_CFLAGS:=$(shell $(PKG_CONFIG) libpng --cflags)
+
+
 ifeq ($(hack),1)
 hack:
 	@echo 'SDL_PANGO_LIB is' $(SDL_PANGO_LIB)
@@ -715,6 +718,7 @@ clean:
 	@-rm -f TuxPaint.dmg temp.dmg; rm -rf magic/*.dSYM Resources
 	@-rm -f dlllist a.exe
 	@-rm -f win32/Preprocessed.iss win32/tuxpaint-*.zip win32/tuxpaint-*.exe
+	@-rm -f test-png
 	@echo
 
 # "make uninstall" should remove the various parts from their
@@ -1400,3 +1404,7 @@ $(MAGIC_SO): magic/%.$(SO_TYPE): magic/src/%.c
 
 .PHONY: magic-plugins
 magic-plugins:	src/tp_magic_api.h $(MAGIC_SO)
+
+test-png:	src/test-png.c
+	$(CC) $(PNG_CFLAGS) src/test-png.c -o test-png $(PNG)
+

docs/CHANGES.txt

@@ -91,6 +91,12 @@ http://www.tuxpaint.org/
       Terrence Sheflin <terrence.sheflin@gmail.com>
       Pere Pujal i Carabantes <perepujal@gmail.com>
 
+    * New tool, "test-png", which exercises libpng library on
+      a set of PNG image files, allowing us to find those which
+      cause Tux Paint (via SDL_image, in turn via libpng) to
+      report back warnings to STDERR.
+      h/t Tim Dickson, per https://sourceforge.net/p/tuxpaint/bugs/252/
+
   * Documentation updates:
     ---------------------
     * Update macOS build instructions for SDL2.0.

src/test-png.c

@@ -0,0 +1,96 @@
+/*
+  test-png.c
+
+  A tiny test program that opens a PNG file using libpng
+  library.  If any warnings come back, this can help us tell
+  when any images include oddities that would cause Tux Paint
+  to echo warnings (via SDL_image->libpng) to stderr when it
+  goes to load them.
+
+  See https://sourceforge.net/p/tuxpaint/bugs/252/
+
+  Bill Kendrick <bill@newbreedsoftware.com>
+
+  (Based loosely on example code by Yoshimasa Niwa, https://gist.github.com/niw,
+  located at https://gist.github.com/niw/5963798, which itself was based on
+  code by Guillaume Cottenceau found at http://zarb.org/~gc/html/libpng.html)
+
+  2022-07-03 - 2022-07-03
+*/
+
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <png.h>
+
+int main(int argc, char * argv[]) {
+  int i;
+  FILE * fi;
+  png_structp png;
+  png_infop info;
+
+  /* Usage output */
+  if (argc == 1 || strcmp(argv[1], "--help") == 0) {
+    fprintf(stderr, "Usage: %s file.png [file.png ...]\n", argv[0]);
+    exit(1);
+  }
+
+
+  /* Redirect stderr to stdout, so libpng's warnings can
+     be in the same stream as our's (where we report each
+     file's filename); allows more easily dumping to a file,
+     piping through `| less`, etc. */
+  dup2(STDOUT_FILENO, STDERR_FILENO);
+
+
+  /* Open each PNG image!... */
+  for (i = 1; i < argc; i++) {
+    printf("%5d ------------------------------------------------------------------\n", i);
+    printf("%s\n", argv[i]);
+
+    /* Open the file */
+    fi = fopen(argv[i], "rb");
+    if (fi == NULL) {
+      printf("Cannot open\n");
+    } else {
+      /* Prepare PNG library stuff... */
+      png = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
+      if (!png) {
+        fprintf(stderr, "Cannot png_create_read_struct()!\n");
+        exit(1);
+      }
+
+      info = png_create_info_struct(png);
+      if (!info) {
+        fprintf(stderr, "Cannot png_create_info_struct()!\n");
+        exit(1);
+      }
+
+      if (setjmp(png_jmpbuf(png))) {
+        fprintf(stderr, "Cannot setjmp(png_jmpbuf(png)))!\n");
+        exit(1);
+      }
+
+      /* Read the PNG's info */
+      png_init_io(png, fi);
+      png_read_info(png, info);
+
+      /* *** At this point, libpng would have reported issues *** */
+
+      /* (We could do additional things, like reporting image
+         dimensions, color depth, etc., but I don't see any reason to
+         care right now.  -bjk 2022.07.03) */
+
+      /* Close and move on to the next file */
+      png_destroy_read_struct(&png, &info, NULL);
+      fclose(fi);
+    }
+
+    printf("\n");
+    fflush(stdout);
+  }
+
+  return(0);
+}