In practice this gives us a context that is *compatible* with GL 2.1. On
my machine it gives a GL 3.3 or 4.3 compatibility profile context,
depending on graphics card installed.
Also fixed enum for core profile (not used yet).
Also added option for GL 3.2 compatibility profile. This will be useful
during Blender 2.8 development, until we are able to use the core
profile. On my machine this gives exactly a GL 3.2 compatibility profile
context, not 3.3 or 4.
Mike Erwin
1858823d3b