wgl/d3d12: Improve initialization fallback logic
This series does 2 things to address #4022 (closed):
- When initializing the Windows libgl-gdi target, if the default driver screen creation fails, fall back to the next option in the sorted list instead of all the way back to softpipe. Don't do this if
GALLIUM_DRIVER
is specified. - If the
d3d12
driver is being initialized, andD3D12_DEBUG
doesn't include "experimental", fail screen creation if the validation tools can't be initialized (i.e. if dxil.dll isn't present). This will trigger an appropriate fallback to a software screen.