opengl.c: added OS X specific stuff, this includes:

 opengl.c: added OS X specific stuff, this includes:
  + support for YUY2 textures
  + support for non-power-of-2 sizes
  + support for VRAM or AGP texturing

 It now uses two buffers and textures (needed for VRAM/AGP texturing).

 I got to fix a few things on the OS X GL provider before I commit it,
 but in the meantime, people can check if I didn't break the opengl
 output on Win32/GLX ;)