MX (Multiple conteXt) support was dropped from the GLEW 2.0 library to make core profile support cleaner.
Our WITH_GLEW_MX build option was OFF by default already; this commit removes the inactive code paths.
I'm working on a plan for multiple GPUs, contexts, resource sharing, etc. This commit gives us a cleaner starting point for that upcoming work.
Tested on Mac, will test on Linux & Windows immediately after pushing.
