Correctly create GLES contexts with GLX/WGL

Earlier commits reworked the necessity of ARB_create_context, creating a few bugs. This series addresses the issues and adds functional tests, to ensure they don't happen again.

The last commit adds a workaround for GLX+OpenGL version test, as needed on one of my machines. I'm suspecting the issue to be specific to my setup, but haven't quite tracked it down.

This supersedes !58 (closed) Fixes #50 (closed)