If an effect is active and it overrides the paint volume, we should
always recompute the paint volume when requested and not use the
cache, since the paint volume override can change from call to
call depending on what phase of painting we are in. For instance,
if we are part way through painting effects and request the
paint volume, the paint volume should only go up to the current
effect, but in a later call to compute repaint regions, the
paint volume needs to expand to accomadate the effect.
This still involves a lot of recomputation in the case of effects -
in a later clutter version it would be worth adding an API to
allow effects to explicitly recompute and return a new the paint
volume up to the current effect as opposed to recomputing
the cached one.
