CHECK_GL_ERROR additions

[revmischa]

Maybe helpful for debugging bad GL calls

[kblaschke]

Before merging and reworking this code again, I'd suggest leaving it in a branch to fix the GStreamer issues, and then implement a proper debugging API via this issue here:

#732

Thst'd save us from reworking any existing code and use a proper debug class/functionality so an outside app can properly log any errors.

[kblaschke]

I guess all the GL_INVALID_ENUM errors seen were due to that uninitialized framebuffer texture being used before the first frame was drawn. This is fixed in PRs #826 and #827.

May I close this PR and postpone proper GL error checking/logging to the implementation of issue #732?

[revmischa]

Sure thing