Support startVertex/startInstance on DX12

[kvark]

Is your feature request related to a problem? Please describe.
DX12 doesn't consider firstVertex/fistInstance to be included into SV_VertexID and SV_InstanceID semantic, but WebGPU requires this.

Describe the solution you'd like
Allocate 2 root constants in front of the signature, then have the shader glue these constants with builtin semantics when asked.
This extra functionality needs to be configurable, since there is a cost to having 2 root constants allocated for each pipeline.

Describe alternatives you've considered
Passing in buffers?

Additional context
We have a test for this, which is quite nice!