Allow creating LayerMaps and using custom area rather than always output size

[YaLTeR]

It would be useful to get non_exclusive_zone() only taking into account specific layers. In particular, I'm thinking about "only overlay and top layer" non-exclusive zone: the idea is to render background and bottom layer popups below the top and the overlay layer, so unconstraining them should take into account only the top and the overlay layer surfaces, but not the background and bottom layer surfaces.

[YaLTeR]

Hm, I guess this doesn't make much sense unless we also add exclusive zone ordering based on the layer. So e.g. top layers always get laid out before (and thus closer to monitor edges) than any bottom layers. Right now the ordering seems to be based on the order of creating the layer surfaces.

[YaLTeR]

After thinking some more about it, I think this is better solved at compositor side by creating and managing separate LayerMaps as necessary rather than one global layer_map_for_output(). Which is not currently possible actually.