compositor-drm: Make scanout view preparation more stringent

Don't import buffers which span multiple outputs, short-cut any attempt
to import SHM buffers, and ignore buffers with a global alpha set.

I'm not convinced all of these conditions entirely make sense, but this
at least makes them equally nonsensical.

Differential Revision: https://phabricator.freedesktop.org/D1414

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
1 file changed