mint-arena: Allow disabling drawing sky for refdef

Used when sky is drawn separate from world, aka skybox portal.
Bunch of comment fixes
mint-arena: Add skybox portal support

Based on RTCW MP. Tested using RTCW's mp_castle map.
mint-arena: Don't warn about props_skyportal in Game VM

It's loaded by CGame VM.
Merge remote-tracking branch 'mint-arena/master'

Conflicts:
	code/cgame/cg_main.c
	code/cgame/cg_view.c
	code/game/g_spawn.c