GLVND support

[dcommander]

Seems like we might be able to solve some problems by tapping into the GLVND interface on modern systems rather than trying to hijack GLX API calls. Further investigation is needed.

[dcommander]

This idea is part of an ongoing quest to eliminate the need for LD_PRELOAD. Conceptually, we could create libGLX_virtualgl.so.0.0.0 and then use the GLVND interface to optionally direct GLX commands from applications to that GLVND library. However, the problem of how to interpose the needed OpenGL and X11 and XCB and (as of VGL 2.6.3) OpenCL function calls remains.