vkd3d: RegisterSpace != 0 is not handled.

[Hans-Kristian Arntzen]

Hi. I was trying to get Control to run in DX12 mode, and the game tries 
to create a PSO where a descriptor table has RegisterSpace != 0.
vk_binding_from_d3d12_descriptor_range() complains about this and 
CreateRootSignature fails as a result.

Is this a simple TODO which just hasn't been needed up until now, or 
will this be very difficult to support?

Cheers,
Hans-Kristian