Add initial wayland support

This pull request adds support for Wayland through EGL.
All wayland only specific routines are placed in wayland_common.
This makes it easier to write other video outputs with the wayland backend.

The EGL specific parts, as well as opengl context creation, are in gl_common.

This backend works for:
 * opengl-old
 * opengl
 * opengl-hq

To use it just specify the wayland backend for opengl
```
--vo=opengl:backend=wayland
```
or disable the x11 build.

Co-Author: Scott Moreau
(Sorry I lost the old commit history due to the file structure changes)