Log Message:
Changes 1.7.38:
* Bail out of a trial if byte count exceeds best byte count so far.
* Added -bail and -nobail options.  Use -nobail to get a complete report
  of filesizes.
* Added -blacken option, to enable changing the color samples of any
  fully-transparent pixels to zero in PNG files with color-type 4 or 6,
  potentially improving their compressibility.