Add support for vertex and pixel shaders

I had started to play with this in a branch before merging the repo to LLVM (https://github.com/llvm/offload-test-suite/tree/cbieneman/graphics). Some of the base pieces (like the buffer reworking) have already landed, and we have image output and comparison support.

We will need to do more work to get graphics pipelines supported and integrated in a cleaner and more sane way.