Tile wrapping

[kkaefer]

For low zoom levels, the world should wrap.

[incanus]

This will be huge on iOS, where our current technical implementation makes this basically impossible.

[kkaefer]

The way to go here is to rename the current Tile object to TileData and create a new Tile object, that just holds a shared pointer to a TileData object. Then, we create Tile objects for the wrapped tile coordinates and assign the appropriate TileData pointer. I believe this is very similar to what we do in JS.