Extend GLEW API to support glewContextInit - GL without implied GLX, …

[nigels-com]

For Issue #172

Documentation needs updating

[mp3guy]

Just wondering if this will make it into master soon?

[Jupeyy]

Hey @nigels-com,
I tested this change under Windows, wayland and X11 and it works, if you only require OpenGL extensions,
maybe put in the documentation that this is intended to use with OpenGL extensions only(e.g. not to initialize EGL stuff etc.) and merge it, thus using it with SDL or other start libraries is the usual use case.
They are more and more wayland users and many distros still don't have 2 versions of glew (egl and glx)

Update:
Someone from the community also tested it on macOS and it worked