[RFC] Avoiding some of the globals in glretrace

[Трухин А.]

Hello,

A bit of testing profiling with several passes revealed that globals
indeed play a role in consistency of results. Though, the
discrepancies are minor -- looks like it affects only the very
beginning of the trace (I guess resetting visuals would be enough to
avoid this actually).

Nevertheless, I decided to try and remove some of globals (in window
helpers and interfaces). I also think this can "improve" retracing
several files (in one run) and make code generally more portable.

Link to the branch: https://github.com/trtt/apitrace/commits/rmglob

Brief summary:
1) Promote callbacks to std::function
2) Wrap GL window system helpers in class
3) Wrap glx, wgl etc. interfaces in classes (register callbacks with
std::bind to the class object)
4) Instantiate Retracer object in the local scope of main loop
5) Instantiate and delete all the above every loop iteration

What do you think?