Log Message:
Changes 0.6.2:
* Upgraded libpng to version 1.2.33-optipng [private]
* Put back a speed optimization, accidentally removed in version 0.6,
  allowing singleton trials (-o1) to be bypassed in certain conditions.
* Fixed an array overflow in the BMP reader.
* Fixed the loss of private chunks under the -snip option.
* Produced a more concise on-screen output in the non-verbose mode.
* Added a programming interface to the optimization engine, in order
  to facilitate the development of PNG-optimizing GUI apps and plugins.