OpenGL attrib_list support

[Alexander Monakov]

Looks awesome.  Quick question -- is handling Xlib-style attrib-lists where
some keys stand alone without a corresponding value possible/easy?  This
should not block the patch, I'm just asking the question while the code is
hot (and I'm interested to have this functionality work for glXChooseVisual).

( By the way, I feel that the mailing list is a better medium for submitting
  patch series of this complexity, but having a github link is nice for easy
  pulling.  Jose, can you clarify the submission policy/guidelines in this
  regard? )

Thanks.

Alexander