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
Use an array of bindtypes rather than the previous situation, which was
a choice of buffer counts, or a heavier builder pattern.
The main thing this unlocks is distinguishing between readonly and
read/write buffers, which is important for DX12.
This is WIP, the Metal part hasn't been done, and the old stuff not
deleted.
Part of #125
Have a structured way of gathering test results, rather than the
existing ad hoc approach of just printing stuff.
The details are still pretty primitive, but there's room to grow.
