Initial version for FBX layered textures.

Fixed assert during parsing UDP3DSMAX.
Initial support for layered textures.
Parts of TrySetTextureProperties for textures and layered textures are common and I think it could be refactored into a single function.

((LayeredTexture*)layeredTexture)->fillTexture(doc); 

This cast is really ugly. Could not solve dependency of textures to layered textures otherwise. Needs refactoring.