UDMF sector property for picking sky

[Kinsie]

This came up recently on a Discord I'm on: It seems like the only way to change between multiple skies is to put a SkyPicker actor in each effected sector, which can be a pain in the butt if you've got hundreds of sectors to deal with (for a large detailed environment, for example). It might be worth adding a UDMF property to sectors to make it easier to handle such circumstances, sort of like how a lot of other visual effects like floor reflectivity are now handled.

If there's an easier way, feel free to point it out and make fun of me. I'm just the messenger in this case.

[Scuba Steve]

It really is a pain in the ass to use multiple skyboxes in complex maps. It should be as simple as defining a skybox in gldefs and using tags to assign that sky to sectors.