The MSL translation of the prefix example had its bindings permuted; a
flag prevents this (but, as is typical for shader translation,
potentially creates other problems).
Also use explicit unsigned literal to avoid DXC warnings.
Add a clear stage and associated tests, and also use it on non-Vulkan
backends to clear the state buffer.
While that's a workaround and will go away when we implement the actual
clear command, it's also a nice demo of how the new "stage" structure
composes.
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.
