Support DX11 on Downlevel Hardware

[cwfitzgerald]

DX11 can work on graphics cards that support DX9-DX11. FL9_1-FL10_1 hardware is referred to by D3D documentation as "downlevel" and adds extra limitations to the api that corresponds to hardware capability. Obviously arbitrary applications won't be able to run on downlevel hardware, but applications written with wide support in mind should be able to use the DX11 backend to reach older hardware without being limited by the backend itself.

Most of us will have hardware that supports FL11_1 or greater, so you can simulate downlevel hardware by going to dxcpl.exe, selecting an exe to set settings for, then selecting a feature level limit.

As of the time of writing, FL11_0 is the lowest feature level that works. The following is a list of issues while running on downlevel hardware:

• [ ] Builtin shaders use compute shaders; not required on pre FL11_0.
• [ ] Some limits/features are still too lenient. https://docs.microsoft.com/en-us/windows/win32/direct3d11/d3d11-graphics-reference-10level9 details changes when running on downlevel hardware.