Base package without ImageSharp

[cyanfish]

Would you be open to splitting the PdfSharpCore package so that the ImageSharp dependency is decoupled?

For example:

• Move ImageSharpImageSource & ImageSharp dependency into a new PdfSharpCore.ImageSharp project that references the PdfSharpCore project
• PdfSharpCore project -> PdfSharpCore.Base Nuget package
  • Keeping old project/folder name to not break all PRs/forks
• PdfSharpCore.ImageSharp project -> PdfSharpCore Nuget package
  • Keeping Nuget package the same so as to not break existing users

For my own uses I would like to use PdfSharpCore with my own IImageSource implementation (currently I use a fork without ImageSharp). I think this would also help resolve the various complaints about ImageSharp v3 as someone could create their own package on top of PdfSharpCore.Base including ImageSharp v3 support.

The only implementation issue I see is this direct dependency from XImage -> ImageSharpImageSource:

PdfSharpCore/PdfSharpCore/Drawing/XImage.cs

Line 82 in cdf089b

if (ImageSource.ImageSourceImpl == null) ImageSource.ImageSourceImpl = new ImageSharpImageSource<Rgba32>();

That could either be replaced with reflection or a separate static helper class (e.g. XImageSharp.FromFile instead of XImage.FromFile). Neither solution is !00% perfect as reflection might not work with trimming and a separate class is obviously a breaking change (though a minor one).

Anyway, if you're open to the idea, I'd be happy to make a PR for this.