Make compositor handle subsurfaces with shm buffers.
Make compositor handle subsurfaces with shm buffers.

So we'r lying for now for shm buffers, and assuming they
have allways been displayed.

Also, this patch fixes so that if you attach a buffer, and the last
buffer has not gotten any damages, then it will be released.

Change-Id: Id1b9789aa8e4dcff75147f6c96220039defc0d8f
Sanity-Review: Qt Sanity Bot <5581206bb7e0307f0d99eb71898ae6b694149ca5@ovi.com>
Reviewed-by: Paul Olav Tvete <97541fb6312d77f06e19078c7c70b2c1f75fb828@nokia.com>

Tofee/qtwayland,locusf/qtwayland-1,ntanibata/qtwayland,locusf/qtwayland-1,Tofee/qtwayland,ntanibata/qtwayland,locusf/qtwayland-1,locusf/qtwayland-1,Tofee/qtwayland,ntanibata/qtwayland