Support for the GL_UNSIGNED_INT_5_9_9_9_REV "type".
Use macros for 5999 constants and for pow(2,x).
Eliminate warnings on Linux.

Interesting the warnings that came when POW2() was a macro.  Linux likes it
better as a function.
Merge pull request #71 from IanAtLunarG/pxfmt_140519_5999Rev

Support for the GL_UNSIGNED_INT_5_9_9_9_REV "type".