[FEATURE] SKMultiPictureDocument support

[antongit]

Is your feature request related to a problem? Please describe.

It would be great if we'll be able to write Multi-Picture Docs (.mskp), but SKMultiPictureDocument is missing from the C# binding. If I'm not missing something. Thank you!

Describe the solution you'd like

Call MultiPictureDocument with an array of SKPictures.

Additional context

https://github.com/google/skia/blob/main/src/utils/SkMultiPictureDocument.cpp