compositor: add early wl_buffer.release

compositor: add early wl_buffer.release

A client can reliably avoid allocating a second buffer per surface, if
the compositor sends the wl_buffer.release event before the frame
callback. To enable clients' single-buffering, release the wl_buffer
early if possible. Otherwise clients will double-buffer.

Releasing early is not possible, if the backend needs the buffer for
migrating a surface to or from a non-primary weston_plane. In that case,
a new buffer must arrive, before the old can be released. Backends will
indicate this by setting weston_surface:keep_buffer to 1 in
assign_planes().

A proper buffer reference in the backends would be better than the
keep_buffer flag, but that would require a per-surface backend private.

The rpi and DRM backends are updated to set keep_buffer, other backends
do not support planes, so do not have to set it.

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