Changes 1.6.7:
  Moved PNG_UINT_CHNK and some other defines from pngcrush.h to pngcrush.c
  Reject invalid color_type or bit_depth.

Changes 1.6.6:
  Added dSIG support.  Pngcrush will not rewrite an image containing
  a dSIG chunk immediately following the IHDR chunk, unless the
  dSIG is explicitly removed with "-rem dSIG" or explicitly kept
  with "-keep dSIG".  In the latter case the saved dSIG chunks will
  become invalid if any changes are made to the datastream.
  Fixed bug in writing unknown chunks from the end_info_ptr.

Changes 1.6.5:
  Discontinued adding a new gAMA chunk when writing sRGB chunk.

(adam)