This adds both regular and Vulkan memory model atomic versions of the
prefix sum test, compiled by #ifdef. The build chain is getting messy,
but I think it's important to test this stuff.
Integrate DXC for translating HLSL for use in DX12. This will work
around FXC limitations and unlock the use of more advanced HLSL features
such as subgroups.
This hardcodes the use of DXIL, but it could be adapted (with a bit of
effort) to choose between DXIL and HLSL at runtime.
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.
