New sky texture tracking system

Allows each game mode and mission to define the set of flats considered
to be sky. Added skies for PSX; this'll also come in handy for Hexen and
Strife later.
Additional static line scroller functions

Up, down, and down fast types for PSX and Strife.
Additional PSX lighting sector types

Some require some timing tweaks still.
Tweak to PSX sector light scale.

No, I'm still not happy with it...