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.
There's a bit of reorganizing as well. Shader stages are made available
from piet-gpu to the test rig, config is now a proper structure
(marshaled with bytemuck).
This commit just has the transform stage, which is a simple monoid scan
of affine transforms.
Progress toward #119
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
