Bug 562: Collision detection for unisolid tiles doesn't handle tilemap offset

Bug 562: Collision detection for unisolid tiles doesn't handle tilemap offset

- Use the new TileMap::get_tile_bbox function, which handles offset.

Cleanups:

- Get the bottom from the original MovingObject rather than subtracting
  movement from dest (an ugly kludge).

- Apply De Morgan's law so the test more clearly corresponds to the
  comment.

Resolves #562. Thanks to Matt McCutchen for updating the patch.

git-svn-id: http://supertux.lethargik.org/svn/supertux/trunk/supertux@6299 837edb03-e0f3-0310-88ca-d4d4e8b29345