Multi-tileset Layers

[DelinWorks]

@aismann (#617) @balancedwolf was trying to add support for multi-tileset layers but didn't have a response from a long time ago and is likely abandoned the PR.

I will be working on it. my way around it will be to split TMXLayer into TMXSubLayer a layer can have one or more sub-Layers, the only thing that will have a render command is a TMXSubLayer each sub-Layer will have it's own tileset, this implementation will be hidden from the user using your typical TMXLayer, when a layer is visited it will render all of it's sub-Layers. I don't know if this is the most efficient way to solve this problem, also the animation system has to be rewritten, it's very inefficient. the animation would be implemented in such a way that it changes all tiles with the same animation all at once, rather than calling SetupTiles and setTileGIDAt each frame for each tile which is quite expensive!

Any suggestions before I start would be awesome!

[aismann]

@DelinWorks
Let its start.