Fixes sprinkled here and there

In this update, I've added lightable fire sources, interactive grass and background walls and features. I've also redone the collision system, so the character no longer randomly bumps up on plain ground. Aaaand I've done some boring optimization work, so the game doesn't lag for no reason.
Have fun!

Now off I go adding a background landscape, player animations and some more mini-levels for the random world generator to use.