Support for embedded image into tilesets.

[IsometricShahil]

Tiled has an option to embed a tileset into the map itself.
The embedded png is injected as a property into the tileset at key "embedded_png".
My suggestion is that cartographer should try to load the embedded png first and then if it doesn't exist, it would fall back to loading the image from the path specified in the .image of the tileset.