Add support for using WARP software rasterizer with the D3D11 backend

[kg]

by setting FNA3D_D3D11_USE_WARP=1

WARP software rasterizer ships with Windows and is fast enough on multicore machines to be useful as a debugging tool (to rule out driver issues, etc) or even as a manual fallback if the user has a bad GPU.

See https://learn.microsoft.com/en-us/windows/win32/direct3darticles/directx-warp and https://learn.microsoft.com/en-us/windows/win32/direct3d11/overviews-direct3d-11-devices-create-warp

[flibitijibibo]

A few changes to shrink the diffs via SDL_GetHintBoolean, otherwise fine!

[Popax21]

The refactoring changes in 37ec1d9 seemed to have introduced a severe bug - see #179.

[kg]

Good catch @Popax21 , I didn't notice it since I didn't do a fresh local build after applying the code review suggestions.

[flibitijibibo]

Will merge the fix as soon as it's submitted!

[Popax21]

Will merge the fix as soon as it's submitted!

Well, the fix is just swapping the options around for both ternaries. Since it's such a simple fix I was under the impression that going through a PR might be more effort from the overhead, however if it's easier for you I can quickly create one.

[flibitijibibo]

Go for it, I'll be in the office in a bit so I can merge and then rebuild fnalibs as soon as I'm in!

[Popax21]

Go for it, I'll be in the office in a bit so I can merge and then rebuild fnalibs as soon as I'm in!

Dang, seems like I missed the time window by just a bit .-. (context: commit fcfab15 just fixed this)