glamor: Lift EGL vendor from XWayland to core Glamor and use it in Xorg.

This is a stripped down !914 (merged) (removed controversal non-db commit, and only rewieved commits remained). For hope of faster merging.

Short summary:

1. It is DRI-independent GLX provider by @ajax for Glamor-based rendering. It is just XWayland's GLX moved into core glamor.
2. I added correct init sequence for modesetting driver to this provider and tested it via glmark2, glfw tests and glxgears. All seems to work without using DRI on Xorg.

Difference between this and !914 (merged) - here is no Xephyr, and no non-db change.

Reviewed-by: Adam Jackson ajax@redhat.com
Acked-by: Emma Anholt emma@anholt.net

CC: @anholt @ofourdan @daenzer @ajax