Improving data storage in TileMap

[b-guild]

I have replaced the hash map of tile map with a resource that stores tiles in 16x16 chunks. This allows the renderer to iterate only the cells of the visible chunks instead of iterating through every tile.

The chunks are stored in a hash map with keys based on the position of the chunk's origin, and the tiles within the chunk are stored in a 16x16 array. Chunk has a custom Visit implementation that stores the handles in a BinaryBlob for space efficiency. The TileMapData resource has a custom Visit implementation that removes empty chunks before writing the data to the visitor.

A TileDefinitionHandle has been designated to represent an empty cell. This should have other uses beyond tile map storage.