Multiple tilesets per tiled layer

Two releases in few days!

This new release 2.11 brings a single but important new feature: now each tiled layer can be made of up to 8 different tilesets.

Prior to this release, each tiled layer had a single reference to an associated tileset. So all tiles used by a layer must be contained in a single tileset. Well, not anymore. Tiled editor has always allowed to use multiple tilesets on each layer, so now you can load maps using more than one tileset on a given tiled layer. New API functions have been implemented to manipulate them in realtime, but for most developers it's enough to let the loader take care of it without any intervention.

Due to performance and architectural considerations, this feature has two limitations that shouldn't be a problem on real projects:

• Up to 8 tilesets per layer
• All tilesets used on a given layer must have the same tile dimensions. You can't mix 8x8 and 16x16 tilesets on the same layer, for example

This release is API/ABI compatible with previous releases, no breaking changes made.