Check out TileMap performance #257

There are two areas where TileMap performance suffers.

1. Drawing off-screen objects. The game doesn't discard off-screen tiles. There are several avenues to implement this.
2. Tile corner calculation. Particularly in WebGL, the game does a lot more matrix calculation than it has to. I think we can be very smart about this with some lerps.

Investigation inspired by Garudaji in this thread: http://www.kiwijs.org/topic/tilemap-online-editor/