DrawFloorAndCeiling is horrendously slow

[Braden Obrzut]

Cause of slowness seems to be tile lookups (assuming the tile never changes gives about a 30fps boost). Needs to be optimized to work more like Doom.

[Braden Obrzut]

Marking as resolved since a cheap relatively modern system can do 1600x1200 @ 70fps, which is good enough to call this a non-issue until the code gets rewritten for other reasons. :)