Change shader building pipeline

[robbiesri]

Right now, we use add_custom_command to build shaders. However, add_custom_command doesn't care about dependencies triggering shader compiles (e.g. modifying a header).

I'm not entirely sure what the right solution is here. I was thinking of using add_custom_target, but that just builds all the time, which I don't want either. I don't want to just treat it as HLSL, because I can't control the DXC compiler, and that doesn't work for Ninja. Which maybe isn't a huge deal.

[robbiesri]

FYI: This makes this a real graphics project, because it's run into this stupid problem