Add deep clone support and picture manipulation in code

[StefanKoell]

Summary:

• Add deep clone support for SKPicture and use it for SKSvg.Clone() so clones are independent of the original model state.
• Make SKSvg.Picture lazy-initialized from Model and avoid setting it during clone operations.
• Add SvgSource.Clone() that preserves original path/stream for ReLoad without eagerly copying the picture.

Limitations:

• Reloading or re-applying CSS loads the model from the original source, so any in-memory modifications to the model are lost.

Result (sample app -> Cloning):

• In the sample app, creating a clone from an SvgImage and applying a grayscale color transform updates the clone while the original stays unchanged, demonstrating independent render state and lazy picture creation.

Sample Code:

        var svgImage = svgCloningOriginal.Source as SvgImage;
        var clone = new SvgImage { Source = svgImage?.Source?.Clone() };

        foreach (var cmd in clone.Source?.Svg?.Model?.Commands?.OfType<DrawPathCanvasCommand>() ?? [])
        {
            var paint = cmd.Paint;
            if (paint?.Color is not null)
            {
                paint.Color = ToGrayscale(paint.Color.Value);
            }

            if (paint?.Shader is ColorShader shader)
            {
                paint.Shader = SKShader.CreateColor(ToGrayscale(paint.Color.Value), shader.ColorSpace);
            }
        }

        clone.Source?.RebuildFromModel();
        svgCloningClone.Source = clone;

[wieslawsoltes]

Alternative implementation:

• Add clone support #454
• Rebuild from model #455

[wieslawsoltes]

Alternative was implemented and merged.