Initial slope implementation (Patreon)

I experimented with a range of implementation approaches. It's difficult finding a good approach which doesn't mess with other physics systems, like external forces or gravity. If I was just making this for myself I could of course chop and change it, but doing that here could mess with peoples games.

Right now it's messy at the apex of platforms (you can hear all the splats from the ground disconnects). The fix is to prioritize forward facing slopes and apply the correct downward force. Right now I'm just choosing the first slope hit. I'll push a refined version soon. 

EDIT: I also added pressing down on one-way platforms to drop through.