Add CAS support

[robbiesri]

https://github.com/GPUOpen-Effects/FidelityFX/tree/master/FFX_CAS

[robbiesri]

@apazylbe added initial support!

Some future looking implementation details:

• Control sharpness via GUI
• Support CAS_BETTER_DIAGONALS. I'd recommend compiling this twice (with and without the define) to have the two binaries. Not sure how we'll handle it in the build system yet though... Maybe two HLSL files: cas.hlsl + cas_better_diags.hlsl ?
• Support CAS_PACKED_ONLY?

[apazylbe]

I had a version of this, but building the shaders with the permutations was ugly and I never dared creating a pull request.. I wanted to create something more elegant and then forgot about this. I will dig it up though and create a PR