fix collision teleportation with multiple obstacles

[nasearle]

Players and enemies sometimes teleport when colliding with two or more obstacles that overlap. not sure what the cause is. if we can't fix it we can always just make sure the obstacles don't spawn on each other as a last resort.

[DavidScales]

one comment about obstacles spawning on each other - its actually kinda nice because we get complex structures for free instead of a screen with just plain rectangles, so I guess if we could keep it that would be nice