Add support for shadows

I know this'll need some significant internal changes to be supported - since we'd render a specific shadow pass to a texture target from the light's POV and then sampled, etc etc.

In a perfect world, I'd want something where each shader implementation can define their own handling of shadow mapping rather than it being something internal and uniform - just so that I could replicate something like Unity's URP in one shader and a different shadow mapping technique in another.

No files, tests, or entry points are named. Start by locating the renderer and shader implementation boundaries, then investigate how a light-view shadow pass could be rendered to a texture and sampled. Done would require support for shadow mapping while allowing shader implementations to choose their own technique.