screencating: query for dmabuf availability before we start streaming

As things are right now, we can only do 32bit textures for dmabuf (see
gbm_bo_format in gbm.h). This means that we were lying to our receivers
when we had 24bit textures by then giving a 32bit texture instead.

This changes it so we request a dummy texture before starting and if we
are offered one we assume they're available and offer a 32bits stream
directly (i.e. BGRA).


(cherry picked from commit cea27813)