Top-Down: Improve collision logic for sliding against solid tiles

[wforbes]

Right now, when you collide with a solid tile or entity your movement stops. That's ok if you're moving perpendicular to the surface, but not if you're moving diagonally against it. Logic should be added that checks for this diagonal movement and only restricts the perpendicular vector. The goal is to allow the player to slide against the colliding tile/entity in the parallel direction.