Log Message:
Changes 1.7.33:
* Ignore all ancillary chunks except during the final trial.  This can be
  significantly faster when large ancillary chunks such as iCCP are present.

Changes 1.7.32:
* Fixed bug introduced in 1.7.30: Do not call png_set_check_for_invalid_index()
  when nosave != 0 (otherwise pngcrush crashes with the "-n" option).