It's unclear whether LayerBuffers will always represent the contents of
one Tile and there's only one implementation of this trait.

This will allow the TileGrid to better track the state of each Tile.

This allows Layers to create their own textures instead of relying on
Servo to do the job. Textures now track whether or not they should be
flipped by passing the buffer to the factory method.

This way each Tile can be responsible for creating its own Textures.