Late night commit!

* Seriously reworked a lot of stuff behind the scenes, thanks to Red's comments.
* More consistent behaviour.
* Launching power is now scaled to the slope's angle relative to the wall you hit, so no massive launches when you hit the side of a steep slope.
* The code is a lot messier and the function name doesn't make any sense any more, so I need to clean that up in the morning.
* Need to figure out how to prevent tiny launches that uncurl you when spindashing up against walls.
* Spindashing trails now takes into account vertical momentum as well as horizontal - it was stupid not seeing your spintrail just because you weren't moving very fast horizontally!

No exe uploaded because it's almost 2am.