Graphical improvements needed to 3d maze mode

[jonoxia]

• Draw doors and stairs instead of just black rectangle openings
• Walls to your immediate left and right are currently invisible (because they intersect with the screen at z=0, drawing them does weird things). Need to special case them or something so your peripheral vision isn't broken.
• Enemies need to appear on the ground, not hovering in the air in front of the walls
• There are some weird glitches that appear mid-rotation at certain angles, not sure why.