Slope fixes
This branch fixes the following slope-related physics and rendering bugs:
* Rings in multiplayer stages respawning inside slopes (even despite being able to spawn ABOVE them on map load)
* Player starts spawning players inside slopes
* Elemental flame trails not appearing if a player spindashes UP a slope; see issue #21
* Dying players "jumping" off slopes to the side if they were previously standing on one
* Some issues with FOF slope rendering
* Various issues with sprites displaying through walls adjacent to slopes
Other features added:
* Objectplace now supports slopes (this is Inuyasha's doing)
* Automap in DEVMODE now supports slopes (my doing)
Just making this merge request now rather than later, ~~in case I decide not to make any more fixes for the time being~~ (this branch doesn't seem to want to die lol), and so we can get these merged in as soon as the code's all been checked over.
See merge request !50
Note: Before this change, North and West directions would be returned as ANGLE_90-1 and ANGLE_180-1. This caused the pusher polyobjects in THZ2 to slowly move sideways as a side-effect (and probably caused similar bugs in the past too, these functions have barely been touched in a decade it turns out.)
