Log Message:
Changes 1.5.1beta09:
Enhanced pngvalid, corrected an error in gray_to_rgb, corrected doc error.
  pngvalid contains tests of transforms, which tests are currently disabled
  because they are incompletely tested.  gray_to_rgb was failing to expand
  the bit depth for smaller bit depth images; this seems to be a long
  standing error and resulted, apparently, in invalid output.  The
  documentation did not accurately describe what libpng really does when
  converting RGB to gray.