Allow Xscreen without KMS or without any outputs if it supports glamor

This allows to use a GPU without KMS or without outputs to be used as a regular device
for a regular screen. This even works with render nodes. This allows, for example,
to have arbitrarely many, render node backed, gpu+glamor accelerated, xscreens.
This may be useful for usecases similar to those a dummy driver is usually used for.

Only render nodes should be used more than once, though, because for other cards,
the clients of the correspondign screens for any non-first instance of such a card
seams to be unable to use it with glx and falls back to llvmpipe instead.

Signed-off-by: Daniel Abrecht <public@danielabrecht.ch>