Merge branch 'wall-collision-fix' into 'next'

Wall collision fixes

Fixes included in this branch:
* A fix for a specific crash encountered in a SUGOI map (sound familiar?) caused by bouncing off a wall adjacent to a slope
* A fix for solid midtextures not accounting for Effect 3/Peg Midtexture properly (https://mb.srb2.org/showthread.php?t=41462)
* A fix for solid midtextures not accounting for texture y offsets properly for non-Lower Unpegged/Peg Midtextured midtextures
* A fix for solid midtextures not accounting for "infinite" repeats (Repeat Midtexture + no repeat count set)
* ~~A fix for Effect 4 on Polyobject First Line making that particular linedef's midtexture solid in addition to making planes visible - this is not wanted if you want a polyobject with both visible planes and full intangibility.~~ Apparently they never did this anyway, don't mind me \o/

See merge request !104