tk2d: Fixed source textures included in the build along the atlas.

***This fix breaks backward compatibility!***

The source textures were included in the build because the tk2dSpriteCollection was directly linked to the the tk2dSpineSkeletonDataAsset.

tk2dSpriteCollection keeps reference to the original textures so keeping a reference to this object therefore included the source textures in the build. The fix is to keep a reference to the tk2dSpriteCollectionData instead. tk2dSpriteCollectionData is what was used in the skeleton code anyway.

Thanks a lot!