Clang cleanup

"taking the absolute value of unsigned type 'unsigned int' has no effect"

using abs() on an unsigned should be nop,  replace them with (angle_t)()

Need someone to verify this does not break any game logic

See merge request !63

Ceiling lighting fix

The ceiling lighting linedef special (and anything else that changes ceiling lighting directly for sectors) works in software mode now.

See merge request !62

Fixed how two adjacent FOFs can prevent each others' walls from displaying if they're at least partially covered

Also some miscellaneous tweaks and changes to account for slopes properly

Unless I'm mistaken, scalesteps/heightsteps should be divided by stop-start, not stop-start+1. Revert this commit if that was intentional.