[libpng16] test: Add a compile-time check in pngimage.c to avoid a fa…

…ilure

Disable the check on `interlace_method` inside function `compare_read`
in pngimage.c, if WRITE_INTERLACING is not supported.

If interlaced encoding is disabled inside libpng, the encoded images
are non-interlaced silently and unconditionally.  This commit updates
the image comparison to skip the interlace check in the resultant
image; other behavior is still checked.

This is a cherry-pick of commit d9f13d8
from branch 'libpng18'.

Reviewed-by: Cosmin Truta <ctruta@gmail.com>
Signed-off-by: John Bowler <jbowler@acm.org>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>

contrib/libtests/pngimage.c

@@ -1018,7 +1018,12 @@ compare_read(struct display *dp, int applied_transforms)
    C(height);
    C(bit_depth);
    C(color_type);
-   C(interlace_method);
+#  ifdef PNG_WRITE_INTERLACING_SUPPORTED
+      /* If write interlace has been disabled, the PNG file is still
+       * written correctly, but as a regular (not-interlaced) PNG.
+       */
+      C(interlace_method);
+#  endif
    C(compression_method);
    C(filter_method);