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
- If the
d3d12driver is being initialized, and
D3D12_DEBUGdoesn'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.