deprecate glew-mx in favour of...

[umlaeute]

a couple of years ago, GLEW decided that multicontext-support is not worth the hassle, and consequently have dropped support for it.

the proposed solution (from glew) is to just only every create compatible contexts (so GLEW only needs to be initialized once).

dropping support for glew-mx and switching to glew (single-context) would allow us to use the latest GLEW again.

related issues:

• https://bugs.debian.org/941053
• possibly #204, #257, #263

[umlaeute]

as of today, i think the better way (instead of switching from glew-mx to glew (single context)), is to use another openGL-loader altogether.

i'm currently favouring glad, which seems to work well so far...