Underlay with punch-through hole on the primary plane

[emersion]

Another use-case for underlays (see #7) would be mapping a layer to the underlay while the layer is supposed to be over the primary plane. To do so, a punch-through hole must be created on the primary plane:

This allows placing overlay planes behind the primary plane (as underlays) while still pretending the views put on the underlays are on top of some views on the primary plane. The punch-through hole can only be used for completely opaque content though. If blending with the primary plane would be necessary, then the hw planes must be in the equivalent z-order.

Some more research is needed to figure out whether it's really worth it and how the API could look like.