Adding cstddef header to voglcommon/gl_types.h

* Fixes error: 'ptrdiff_t' does not name a type
Merge pull request #56 from sabus/master

Adding cstddef header to voglcommon/gl_types.h
Merge pull request #58 from vMcJohn/master

Properly emulate opengl32.dll for both win32 and win64
ktxtool cleanup
Merge branch 'master' of https://github.com/ValveSoftware/vogl