wgpu-hal needs a no-op backend for fuzzing and benchmarking

[jimblandy]

It would be nice if there were a wgpu-hal backend that basically does nothing and returns Ok for every request. Even though buffer contents would never be right, and presentation would not happen, this would be useful for fuzzing and benchmarking wgpu-core.

[jimblandy]

The insight here is that the only time you can actually tell whether the GPU did anything at all is when you look at buffer contents. Many many tests don't even bother to do this. And it would be incredibly useful to have the option of cutting the driver and the GPU out of the picture when benchmarking and fuzzing.

[jimblandy]

Much of the work we'll be doing is actually entirely platform-independent, so this would be a portable and fast way to validate that work.