Blittable color

[AlexRadch]

Close #48615

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[tannergooding]

CC. @JeremyKuhne, is WinForms ready to start using these types if exposed?

Adding them has only been blocked as that was the primary user and we didn't want to expose them without WinForms adopting them in the same release

[JeremyKuhne]

@tannergooding I'll be consuming this in WinForms WPF this cycle after it goes in.

[tannergooding]

Was talking with @JeremyKuhne and his expectation matched mine, which was this was going to be part of the existing System.Numerics.Vectors package rather than a net new package.

Particularly for acceleration and support around float, there is likely to be a need for these (the color and existing Vector4/Matrix4x4/etc types) to be used in conjunction. The "downside" is that since System.Numerics.Vectors is in-box and frozen downlevel these new types would never be available on .NET Framework. However, that isn't a concern from the WinForms/WPF team (much as any of the other new graphics related vector/matrix/plane/etc APIs aren't available downlevel).

CC. @ericstj, just for additional confirmation there isn't something we're missing here