Add multi-skin texture support

[chpatton013]

AssImp allows several textures to exist for each channel. Crash currently uses the first textures found in each of the displacement, normal, ambient, diffuse, specular, and shininess texture channels to populate the graphics device. However, if every texture found in each channel were used, the notion of a skin could be created by mapping each channel to the texture index to use for each MeshInstance.