Add auto generating sky box to compiler

[elektr1kOS]

I had an idea from a program called AutoSky, that auto-generates a skybox from a visgroup. This program is helpful in most cases, but it doesn't automatically size down props. One new addition to HammerAddons could be that if you define a VisGroup or group of objects in a class (maybe comp_auto_skybox), that in the compile process (postcompiler), it would take all of the props, decompile them, size them down, maybe prop combine them?, and then compile and put them in the auto-generated skybox.

Note

I don't actually know if this is even possible since AutoSky uses the .vmf and not the .bsp, but I'd like to see this, as this could speed up optimisation a lot.

This could be super helpful for styles like Wheatley or BTS, where if the player never goes outside of the chamber, it could just be a skybox, helping performance on most maps!