Simplify shm buffer handling

Simplify shm buffer handling

There was a lot of code here to do a lot of work we didn't need to do.
If we damage a surface with a shm buffer attached, all we need to do
is to re-upload the damaged region to the texture.  As for drm buffers,
we don't assume anything changes on attach and only update the
regions the client tells us to update in the damage request.