This is a function that's meant to be used whenever both color states
of the shader are equal. In that case no colorspace conversion code
needs to be created and shaders can be shared.
This adds a GdkColorStates that encodes 2 of the default GdkColorStates
and wether their values are premultiplied or not.
Neither do the shaders do anything with this information yet, nor do the
shaders do anything with it yet, this is just the plumbing.