[For discussion] Add support for slang shaders

[SaschaWillems]

With slang now in the hands of Khronos we should discuss if we want to add support for it along with glsl and hlsl. When we added hlsl support, we adjusted the build setup and framework in such a way that adding in another shading language is pretty straight-forward.

If we decide on doing this, I'd be willing to add support for slang to the framework ;)