Add support for opaque types

[davesmith00000]

Original description: Do opaque types with inline methods/extension methods 'just work', and if not, could we add support for them to make reasoning about complicated shader logic easier, e.g. when converting between multiple spatial representations.

Answer is no! You get this error:

ultraviolet.datatypes.ShaderError$Unsupported: [ultraviolet] Shaders do not support references to 'this'.

So it'll need some work.