Implement gliding

Completed the basic logic for gliding, ground movement, jumping, crouching, and transitioning between these states. Right now crouching doesn't actually do anything except set a flag.

I also added a new material type to remove friction from walls - I changed the DirtTile prefab to use this material but we'll need to use it for future materials too (if we plan to have them as walls).

Removed the old GlideTestScene, please use LevelGeneTest scene instead. I left it the same, except added the sprite to it.