window: destroy the 2nd shm buffer, if both released

Handle the case when we the compositor somehow migrates from requiring
double buffering into working on single buffering, so we release the
extra shm buffer.

Currently, I do not think this can happen in practice, but in the future
it may happen with sub-surfaces.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
1 file changed