This commit adds timestamps to compute pass boundaries for command style timer queries on metal.
It also updates the code in piet-gpu/stages, piet-gpu/lib.rs and tests/ to use the new ComputePass type.
Reduce allocation of descriptor heaps. This change also enables clearing
of buffers, as the handles are needed at command dispatch time.
Also updates the tests to use clear_buffers on DX12. Looking forward to
being able to get rid of the compute shader workaround on Metal.
This is a followup on #125, and progress toward #95
This is our version of the standard message passing litmus test for
atomics. It does a bunch in parallel and permutes the reads and writes
extensively, so it's been more sensitive than existing tests.
