fix from upstream for a regression in grey handling

http://bugs.gentoo.org/181318

--- pngrtran.c
+++ pngrtran.c
@@ -1293,9 +1293,11 @@
       if(rgb_error)
       {
          png_ptr->rgb_to_gray_status=1;
-         if(png_ptr->transformations & PNG_RGB_TO_GRAY_WARN)
+         if((png_ptr->transformations & PNG_RGB_TO_GRAY) ==
+             PNG_RGB_TO_GRAY_WARN)
             png_warning(png_ptr, "png_do_rgb_to_gray found nongray pixel");
-         if(png_ptr->transformations & PNG_RGB_TO_GRAY_ERR)
+         if((png_ptr->transformations & PNG_RGB_TO_GRAY) ==
+             PNG_RGB_TO_GRAY_ERR)
             png_error(png_ptr, "png_do_rgb_to_gray found nongray pixel");
       }
    }