Replaced __stdcall with CALLBACK (which glew defines on Windows).

Thanks for fixing the __stdcall error; I completely forgot about using OpenGL on Windows. 

This pull request contains a minor fix for OpenGL on Linux; __stdcall doesn't exist, so I replaced it with CALLBACK (which glew.h defines on Windows), and added a CALLBACK macro in the CPP file for Linux.

With this commit, everything works in GL1 on Linux.