Wayland support

Based on the original port to Wayland by: Joel Teichroeb, Benjamin Franzke, Scott Moreau, et al.

Additional changes in this commit, done by me:

* Wayland uses the common EGL framework
* EGL can now create a desktop OpenGL context
* testgl2 loads GL functions dynamically, no need to link to libGL anymore
* Assorted fixes to the Wayland backend

Tested on the Weston Compositor (v1.0.5) that ships with Ubuntu 13.10,
running Weston under X. Tests ran: testrendercopyex (all backends), testgl2, testgles2,testintersections