Cleanups related to tilemap positions:

Cleanups related to tilemap positions:

- Combine TileMap::{x,y}_offset into a vector TileMap::offset.

- Factor out some widely duplicated code into new TileMap methods
  get_tile_position, get_bbox, get_tile_bbox, get_tiles_overlapping.

- AATriangle contains a Rectf instead of two vectors.

- Some simplification to Sector::is_free_of_tiles.

- BadGuy::might_fall passed a zero-width rectangle to
  Sector::is_free_of_tiles.  With the new code, such a rectangle can slip
  between two tiles without overlapping either one, giving an incorrect
  result.  So change the BadGuy::might_fall rectangle to have width 1.

Thanks to Matt McCutchen for this patch.

SVN-Revision: 6297