This patch includes a number of changes to encourage reading and writing
buffers through mapping rather than copying data as before.
This includes a new `BufWrite` abstraction which is designed for filling
buffers. It behaves much like a Vec<u8>, but with fixed capacity.
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.
