vfb: add DRI3/glamor support

This commit adds DRI3/glamor support, effectively translating into
hardware GPU support.

Theoretically it should be possible to use DRM/GPU drivers such as
virtio or vgem, although only the intel i915 driver is currently tested.

Since Xvfb does no modeset, it opens the render node. Currently that is
fixed to "/dev/dri/renderD128" and will be tweaked with future commits.

Specific use-cases are left for the reader - testing glamor, GL driver
or others.

v2: Drop GLAMOR_NO_XV, use GLAMOR_FOR_XORG instead (Michel Dänzer)
v3: Fix build w/o glamor
 - Split out glamor dependency patch for meson (Pekka)
 - Enhance commit message (Pekka)
 - Use O_CLOEXEC with open() (Pekka)
 - Enhance error path, memory leak comments (Pekka)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
3 jobs for sent/xvfb-render-node in 5 minutes and 12 seconds (queued for 1 second)