This MR has a few things, which I'm happy to split and land separately if people would like:
d3d12 as a software driver option for
GALLIUM_DRIVER similar to zink.
This is the first steps towards #3835 (closed).
Eventually, we'd like to build a system for automatically selecting the
d3d12 driver if
/dev/dxg is present, but that likely requires some X/Wayland work to be able to marshal the contents. Simply using the software driver path should be good enough for now. This MR originally went pretty far down that rabbit hole, but I've cut it back for now to give us something simpler and more functional to merge sooner.