Implement game shaders in 3D viewport

[Kruithne]

Instead of the basic diffuse material that we are using, implement the known shaders from the game client so that our previews are considerably better than how they appear currently.

[Marlamin]

Note that we'll likely limit this to blending operations available in glTF only as we'll be limited to that for exports as well. Extending glTF with unofficial glTF extensions is an option in the future to contain WoW-specific data that glTF does not support that we can then use in the Blender plugin, but that's too far in the future right now to decide on.