This patch implements the Mask node for the realtime compositor. The
evaluation of the mask is not GPU accelerated, but is cached as a form
of temporary implementation until we implement a GPU evaluator for
masks.
One limitation currently is that we do not redraw the viewport while the
mask is getting edited by the user, because always doing that will be
redundant in most situations, and conditioning the redraw requires a lot
of work that should be handled outside of this patch.
This is similar to the Texture node implementation in 151a53110c.
