xwayland/glamor: make wl_drm optional

Chia-I Wu requested to merge olv/xserver:optional-wl-drm into master

This MR adds zwp_linux_dmabuf_feedback_v1 support and makes wl_drm optional. The motivation is that Chrome EXO (a wayland compositor) does not support wl_drm, and it seems zwp_linux_dmabuf_feedback_v1 can obsolete wl_drm.

I am not super familiar with either xwayland or the wayland protocols. Adding folks that I think can help.

/cc @daniels @emersion @leandrohrb @rmader

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