[libpng16] Fixed array size calculations to avoid warnings. At various points

in the code the number of elements in an array is calculated using
sizeof.  This generates a compile time constant of type (size_t) which
is then typically assigned to an (unsigned int) or (int). Some versions
of GCC on 64-bit systems warn about the apparent narrowing, even though
the same compiler does apparently generate the correct, in-range,
numeric constant.  This adds appropriate, safe, casts to make the
warnings go away.

pngread.c

@@ -1619,7 +1619,7 @@ png_image_skip_unused_chunks(png_structrp png_ptr)
 
        /* But do not ignore image data handling chunks */
        png_set_keep_unknown_chunks(png_ptr, PNG_HANDLE_CHUNK_AS_DEFAULT,
-         chunks_to_process, (sizeof chunks_to_process)/5);
+         chunks_to_process, (int)/*SAFE*/(sizeof chunks_to_process)/5);
     }
 }