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
This adds a prefix sum test. This patch is also trying to get a little
more serious about structuring both the test runner (toward the goal of
collecting proper statistics) and pipeline stages for the tests.
Still WIP but giving good results.