Flipping an image changes the bounds of each tile. The existing
code assumes that cannot happen. getTile() calculates the tile
bounds the first time it is asked for a particular tile. It then
caches and returns the same time on every subsequent call.
getTile() has a check to test if a tile exists in the cache. If
it does not, the tile is created and inserted. In order to make
tiles be rebuilt after a flip, we only need to check if the tile's
flip matches the image's flip. If not, we can recreate the tile
as if it did not exist.
To make this a bit clearer, the tile's flipped flag is now set
in getTile() rather than positionTile().
This makes setFlip() work.
