Migrate ImageSharp → SkiaSharp for `ImageSource` + tests & documentation

[mapo80]

Migrate ImageSharp → SkiaSharp for ImageSource + tests & documentation

Summary

This PR replaces the ImageSharp-based implementation with a new SkiaSharpImageSource for raster loading/manipulation and embedding images into PDFs. It also updates/extends the test suite (including SkiaSharp-specific tests) and adds clear build/test instructions with explicit prerequisites.

Why

• Licensing & distribution: SkiaSharp is MIT. ImageSharp uses the Six Labors Split License, which may require a commercial license in some scenarios. Moving to SkiaSharp reduces licensing friction while preserving feature richness.
• Cross-platform reliability: SkiaSharp provides mature bindings to Google’s Skia engine. Native asset packages are referenced to ensure stable behavior on Linux (CI and containers).

What changed

Library

• New image source: Added SkiaSharpImageSource and removed ImageSharpImageSource.
• Integration with XImage: The default ImageSource.ImageSourceImpl now uses SkiaSharpImageSource.
• Package references: Removed SixLabors.ImageSharp; added SkiaSharp and the appropriate native assets package for Linux.

Tests

• New migration tests: Validate dimensions, alpha/transparent pixels, SaveAsJpeg, PDF bitmap embedding paths, and FromFile/FromStream.
• Additional coverage: Loading from raw byte arrays, JPEG quality affecting output size, XImage.FromImageSource, and implicit ImageSourceImpl initialization.
• Rendering-diff tolerance: Introduced a small DiffTolerance (instead of strict == 0) for assertions sensitive to backend rendering nuances, stabilizing tests across platforms.
• Image merge sizing: Relaxed an overly strict size assertion after image consolidation.

Docs & scripts

• README: Added a “SkiaSharp Migration” section explaining rationale and impact.
• AGENTS.md: Build/test instructions (dotnet 8), non-interactive font installation notes, and prerequisites for image-based tests (e.g., Ghostscript, Microsoft TrueType fonts).

Backward compatibility

• Potentially breaking (internal): Projects that referenced ImageSharpImageSource directly should migrate to SkiaSharpImageSource.
• Public surface: High-level APIs (XImage.FromFile/FromStream) remain unchanged; the default image backend underneath is now SkiaSharp.

How to test

# (Ubuntu) prerequisites for image + font tests
sudo apt-get update
sudo apt-get install -y ghostscript ttf-mscorefonts-installer
fc-cache -f -v

# build & test
dotnet build PdfSharpCore.sln
dotnet test --framework net8.0 PdfSharpCore.Test/PdfSharpCore.Test.csproj

Checklist

• [x] Replace ImageSharp with SkiaSharp for the default image backend
• [x] Update package references (SkiaSharp + Linux native assets)
• [x] Add/adjust tests for SkiaSharp and stabilize rendering-sensitive assertions
• [x] Update README and add contributor build/test guidance