Skin Manager additional texture support
Changes to the Skin Manager and references to allow for additional textures to be applied via Skins. Skin Manager now supports Specular/Metallic, Smoothness, Normal Maps, Occlusion, and Emission textures.
Specular/Metallic and Smoothness must be in the same texture, with RGB channels containing specular/metallic data, and the alpha channel containing smoothness data.
Normal maps are not currently supported in the aircraft livery shader, but the skin manager will apply them anyways to create future support.