Supporting the BlendFactor in the D3D backend

[baSSiLL]

I wanted to use the BlendStateDescription.BlendFactor property along with BlendFactor.BlendFactor and BlendFactor.InverseBlendFactor enum values in an attachment. It turned out that this function is not implemented properly for the D3D backend. The value of the blend factor is not passed to OMSetBlendState, and also BlendFactor.InverseBlendFactor is converted to a wrong D3D value.
This pull request fixes the described issue and includes a test.

[baSSiLL]

As the BlendStateDescription.BlendFactor is not nullable, I decided to pass its value to OMSetBlendState always even though the corresponding parameter is nullable. Probably there's some reason to pass null when the blend factor is not used actually but I don't see it. If needed, this could be done by analyzing the XXXFactor properties in attachments for BlendFactor.BlendFactor and BlendFactor.InverseBlendFactor values.

[baSSiLL]

Hi @mellinoe
I see the CI build fails with

C:\Users\runneradmin.nuget\packages\nerdbank.gitversioning\3.3.37\build\Nerdbank.GitVersioning.targets(115,5): error MSB4018: The "Nerdbank.GitVersioning.Tasks.SetCloudBuildVariables" task failed unexpectedly. [D:\a\veldrid\veldrid\src\Veldrid.ImageSharp\Veldrid.ImageSharp.csproj]
C:\Users\runneradmin.nuget\packages\nerdbank.gitversioning\3.3.37\build\Nerdbank.GitVersioning.targets(115,5): error MSB4018: System.IO.IOException: The process cannot access the file 'D:\a_temp_runner_file_commands\set_env_2f27e3bc-4baf-4475-b4b1-06513a2afdd5' because it is being used by another process. [D:\a\veldrid\veldrid\src\Veldrid.ImageSharp\Veldrid.ImageSharp.csproj]

Which doesn't look caused by my commits. I guess this may be a temporary glitch which won't happen on the next build. Apparently I'm not the one who can force CI to rebuild :)

[zaafar]

@baSSiLL push a dummy commit and it will re-trigger the CI

[baSSiLL]

Thanks @zaafar I thought about this too but I don't like the idea to make dummy commits for any reason.
Besides, it won't help if there's some issue with the build environment. So it would be better that an owner took a look.

[zaafar]

Make sense @baSSiLL.

@mellinoe FYI: using the latest version of NerdBank lib fixes this issue.

ref: dotnet/Nerdbank.GitVersioning#556

ref2: dotnet/Nerdbank.GitVersioning#561