Merge branch 'slope-fixes' into 'next'

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