DP GFX SKYBOX
From Quake Wiki
Worldspawn fields:
"sky" (parameters: "basename", example: "mtnsun_" would load "mtnsun_up.tga" and "mtnsun_rt.tga" and similar names, note: "sky" is also used the same way by Quake2)
Global skybox for the map, can not be changed by QuakeC.