test-png: Also read images, to get interlace warnings

Check output for "libpng warning:" lines; the tool also echos
its own "test-png warning" lines about other possible concerns
(e.g. strange bitdepths, etc.) which are not necessarily going
to cause Tux Paint to echo things to STDERR.

src/test-png.c

@@ -9,6 +9,10 @@
 
   See https://sourceforge.net/p/tuxpaint/bugs/252/
 
+  Example:
+    ./test-png data/images/icon.png
+    ./test-png `find . -name "*.png"` | grep -B 2 libpng
+
   Bill Kendrick <bill@newbreedsoftware.com>
 
   (Based loosely on example code by Yoshimasa Niwa, https://gist.github.com/niw,
@@ -26,10 +30,13 @@
 #include <png.h>
 
 int main(int argc, char * argv[]) {
-  int i;
+  int i, w, h, y;
   FILE * fi;
   png_structp png;
   png_infop info;
+  png_byte ctype, depth;
+  png_bytep * rows;
+
 
   /* Usage output */
   if (argc == 1 || strcmp(argv[1], "--help") == 0) {
@@ -78,13 +85,83 @@ int main(int argc, char * argv[]) {
       png_init_io(png, fi);
       png_read_info(png, info);
 
-      /* *** At this point, libpng would have reported issues *** */
+      /* *** At this point, libpng would have reported issues 
+         about some chunks; e.g. "iCCP: too many profiles",
+         "sRGB: out of place", "bKGD: invalid gray level", etc. *** */
 
-      /* (We could do additional things, like reporting image
+      w = png_get_image_width(png, info);
-         dimensions, color depth, etc., but I don't see any reason to
+      h = png_get_image_height(png, info);
-         care right now.  -bjk 2022.07.03) */
+      ctype = png_get_color_type(png, info);
+      depth = png_get_bit_depth(png, info);
+
+      /* If 16-bit, strip to 8-bit */
+      if (depth == 16) {
+        printf("test-png warning: 16-bit\n");
+        png_set_strip_16(png);
+      }
+
+      /* Switch palette to RGB */
+      if (ctype == PNG_COLOR_TYPE_PALETTE) {
+        printf("test-png warning: paletted\n");
+        png_set_palette_to_rgb(png);
+      }
+
+      /* Expand low-depth greyscale up to 8-bit */
+      if (ctype == PNG_COLOR_TYPE_GRAY && depth < 8) {
+        printf("test-png warning: greyscale with only %d-bit depth\n", depth);
+        png_set_expand_gray_1_2_4_to_8(png);
+      }
+
+      /* Expand tRNS chunks into alpha */
+      if (png_get_valid(png, info, PNG_INFO_tRNS)) {
+        printf("test-png warning: contains tRNS chunk\n");
+        png_set_tRNS_to_alpha(png);
+      }
+
+      /* Fill alpha channel if there is none */
+      if (ctype == PNG_COLOR_TYPE_RGB ||
+          ctype == PNG_COLOR_TYPE_GRAY ||
+          ctype == PNG_COLOR_TYPE_PALETTE) {
+        png_set_filler(png, 0xFF, PNG_FILLER_AFTER);
+      }
+
+      /* Expand grey to color */
+      if (ctype == PNG_COLOR_TYPE_GRAY ||
+          ctype == PNG_COLOR_TYPE_GRAY_ALPHA) {
+        printf("test-png warning: greyscale\n");
+        png_set_gray_to_rgb(png);
+      }
+
+      /* Update 'info' struct based on whatever was changed above */
+      png_read_update_info(png, info);
+
+      /* Allocate space */
+      rows = (png_bytep *) malloc(sizeof(png_bytep) * h);
+      if (!rows) {
+        fprintf(stderr, "Failed to malloc() space for image data rows!\n");
+        exit(1);
+      }
+
+      for (y = 0; y < h; y++) {
+        rows[y] = (png_byte *) malloc(png_get_rowbytes(png, info));
+        if (!rows[y]) {
+          fprintf(stderr, "Failed to malloc() space for image data row #%d!\n", y);
+          exit(1);
+        }
+      }
+
+      png_read_image(png, rows);
+
+      /* *** At this point, libpng would have reported issues while
+         loading the image, e.g.
+         "Interlace handling should be turned on when using png_read_image" */
+
+
+      /* Clean up and move on to the next file */
+      for (y = 0; y < h; y++)
+        free(rows[y]);
+      free(rows);
 
-      /* Close and move on to the next file */
       png_destroy_read_struct(&png, &info, NULL);
       fclose(fi);
     }