OpenGL on Mac OS 9

Getting OpenGL going on Mac OS 9 under Retro68, without glut, sucks.

This repo fixes that by:

• Provides the proper stub libraries for use with Retro68 (lib)
• Provides the header files from Apple's original OpenGL SDK (include)
• Provides an example on how to use OpenGL without glut, using Apple's "agl" functions instead (examples)

To build said example:

• set $RETRO68_TOOLCHAIN_PATH to the path of your Retro68 toolchain
• set $RETRO68_INSTALL_PATH to the cloned Retro68 repo.
• run build.sh to build for both 68k and PowerPC.