Use Root Signature 1.1 on DX12

[cwfitzgerald]

Root signature 1.1 includes a series of flags for all descriptor tables and root descriptors that tell you how volatile/static the descriptor and data is. We use descriptors rather conservatively, so this would give the implementation more to optimize with.

See also https://learn.microsoft.com/en-us/windows/win32/direct3d12/root-signature-version-1-1