wayland_shm: Return oldest buffer for new renders

wayland_shm: Return oldest buffer for new renders

This is what the old shm code has been doing, and it's probably better
than what the dmabuf code was doing.

We currently allocate 3 buffers.  The usual case has us swapping between
two of those buffers and saving that third buffer for emergencies - if
we ever need that third buffer it'll require a full redraw.

If we return the oldest available buffer the usual case requires a little
more damage but we should never hit the full redraw case, which can cause
a frame drop on slow hardware.