Add clone support

[wieslawsoltes]

Svg.Skia deep cloning and clone coverage enhancements

Overview

This branch standardizes deep cloning across ShimSkiaSharp and Svg.Model, preserves shared-reference semantics where appropriate, and adds comprehensive clone coverage tests. It introduces a formal clone contract via interfaces, extends drawable cloning across the Svg.Model object graph, and adds unit test projects and coverage checks to validate behavior.

Key changes

• Standardized deep cloning across ShimSkiaSharp with IDeepCloneable<T> and DeepClone() implementations for records and classes, including SKDrawable support.
• Added a clone context with reference-equality tracking and cycle detection, preserving shared-instance identity within a clone graph and rejecting self-referential record graphs.
• Updated ShimSkiaSharp clone paths for canvases, commands, paths, paints, shaders, filters, pictures, images, text blobs, typefaces, clip paths, and recorders to use shared-context cloning.
• Implemented deep-clone support for Svg.Model drawables, including base state copying, container/marker reparenting, and type-specific field cloning.
• Added a new Svg.Model unit test project with clone tests for drawables and coverage checks for clone support.
• Expanded ShimSkiaSharp unit tests to cover shared-reference preservation and cycle-detection behavior.

Behavior changes

• DeepClone now preserves shared object identity within the same clone context rather than duplicating shared references.
• Self-referential record graphs throw NotSupportedException during cloning to avoid infinite recursion.
• Svg.Model drawables now provide consistent deep cloning with preserved parent relationships for cloned child graphs.

Tests

• dotnet test tests/ShimSkiaSharp.UnitTests/ShimSkiaSharp.UnitTests.csproj -c Release
  • Note: existing nullable warnings in tests/ShimSkiaSharp.UnitTests/SKPathTests.cs.
• dotnet test tests/Svg.Model.UnitTests/Svg.Model.UnitTests.csproj -c Release