GL_Init: on OS X, enable multi-threaded OpenGL mode.

This moves some of the OpenGL implementation to a background thread, and makes draw calls block for a shorter amount of time. AFAIK, this is the default mode on most other OS'es, not sure why you have to explicitly ask for it on OS X.

N.B. the kCGLCEMPEngine constant is available on 10.4+ so I don't bother checking the OS X version.
More info: https://developer.apple.com/library/mac/technotes/tn2085/_index.html

git-svn-id: svn+ssh://svn.code.sf.net/p/quakespasm/code/trunk@1137 af15c1b1-3010-417e-b628-4374ebc0bcbd
Merge branch 'master' into alias5