wayland: add initial wayland support

All wayland only specific routines are placed in wayland_common.
This makes it easier to write other video outputs.

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 opengl backend
    --vo=opengl:backend=wayland
or disable the x11 build.

Don't forget to set EGL_PLATFORM to wayland.
wayland: add vo backend description