Make the compositor redraw when a surface gets a commit().

It must redraw, even if it doesn't actually redraw anything, irrespective
of the fact that a attach/damage has come or not. Just calling frame();commit()
is perfectly valid.

Change-Id: If298654b8a7cb7c954cfbad8618f23134731cb9f
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>