¡Cinder! ㊝

System.Drawing is only supported on Windows, well we already knew that, but .NET6 marks classes like `Bitmap` as unsupported outside of Windows. The joy... Convert to using ImageSharp or something...

This adds .nupkg signing via `nuget sign` to Authenticode signing. nuget.exe needs to be installed on not-windows and path set to it. (Nuget still requires mono on other platforms.) I...