Add depth bias to RHI to support Quick3D shadow mapping

https://docs.microsoft.com/en-us/windows/win32/direct3d11/d3d10-graphics-programming-guide-output-merger-stage-depth-bias

Quick 3D renders shadow maps with depth bias 2 and slope-scaled bias 1.5.

Also beware that the bias is int in D3D, whereas others are using float. So QRhi has to go with int as well.

Depth clamp is ignored for now.