Will also fix any other case that an mobj has a skin and a single frame.

walls not extending down when a thok barrier has top textures and both
ceilings have sky.

Avoid Z-Fighting on the depth wall with the regular walls to stop the
possibility of regular walls being made partially invisible.

Gets rid of stuff being drawn where the sky should be being drawn.

sky positioning and scaling more correct compared to software.

I think I've done this all right, someone correct me if I haven't.

see: CEZ1 skybox in linear filtermodes.

less redundancy.

I realise what I did before may cause FOF's in the same sector to not be
drawn, if they were before.

polyobjects do have top/bottom textures, silly.
And translucency.

Remove momentum reduction in CA_JUMPTHOK

Also add new multiability handling for it

    This makes positional audio in sdl work correctly (things to the left
    play sounds in the left speaker)

This makes positional audio in sdl work correctly (things to the left
play sounds in the left speaker)

SDL1.2 is dead, long live SDL

to avoid a crash

Should now ignore all motion events and not warp the mouse when the
window is out of focus.