libweston/compositor: Check whether flushing is allowed

This patch acts as bandaid in the core compositor to avoid the renderer
doing a flush after the buffer has been released. Flushing after release
can happen due to problems in the internal damage tracking, is violating
the protocol, and causes visible glitches.

A more proper fix would be to handle compositor side damage correctly.

Suggested-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Daniel Stone <daniel.stone@collabora.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>