window: implement shm triple-buffering

window: implement shm triple-buffering

Increase the maximum number of shm "leaves" to three, and rewrite the
leaf release and pick algorithms. The new algorithms hopefully improve
on buffer re-use while freeing unused buffers.

The goal of the new release algorithm is to always leave one free leaf
with storage allocated, so that the next redraw could start straight on
it.

The new leaf picking algorithm will prefer a free leaf that already has
some storage allocated, instead of just picking the first free leaf that
may need to allocate a new buffer.

Triple-buffering is especially for sub-surfaces, where the compositor
may have one wl_buffer busy on screen, and another wl_buffer busy in the
sub-surface cached state due to the synchronized commit mode. To be
able to forcibly repaint at that situation for e.g. resize, we need a
third buffer.

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