Handle relative image hrefs without base URI

[wieslawsoltes]

PR Summary: Handle Relative Image Hrefs Without a Base URI

Overview

Fixes SvgService.GetImageUri / image loading behavior for SVG documents that contain a relative image reference, such as xlink:href="6F03BD87.png", when the owning document has no BaseUri.

Previously, unresolved relative image URIs could flow into the external image loading path. In issue #472, this showed up as a null-base URI failure while trying to resolve a relative image reference. The intended behavior for this case is to gracefully skip the missing/unresolvable image and continue rendering the rest of the SVG.

Changes

• Updated SvgService.GetImage so unresolved relative image URIs return null directly.
• Kept data URI handling unchanged.
• Kept absolute URI image loading unchanged.
• Added model-level regression coverage for:
  • Relative image hrefs without a document base URI.
  • GetImage returning null for unresolved relative image hrefs.
  • The document-owner overload preserving relative URI behavior when BaseUri is missing.
  • The document-owner overload resolving relative hrefs when BaseUri exists.
• Added SKSvg rendering regression coverage for an issue-shaped SVG that includes a missing relative xlink:href.
• Strengthened the rendering regression by checking output dimensions and representative rendered pixels, proving normal SVG content still renders after the missing image is skipped.

Rationale

An SVG loaded from a stream or inline string may not have a usable BaseUri. In that case a relative external image reference cannot be resolved safely. Returning null from image loading matches the existing renderer behavior for unavailable images: the image node becomes non-renderable while the rest of the document continues through the render pipeline.

This avoids depending on exception handling from WebRequest.Create or URI construction for normal missing-resource control flow.

Verification

Commands run during development:

git submodule update --init --recursive
dotnet restore Svg.Skia.slnx
dotnet format Svg.Skia.slnx --no-restore --include src/Svg.Model/Services/SvgService.cs tests/Svg.Model.UnitTests/SvgServiceImageUriTests.cs tests/Svg.Skia.UnitTests/SKSvgTests.cs
git diff --check
dotnet test tests/Svg.Model.UnitTests/Svg.Model.UnitTests.csproj -f net10.0 -c Release --no-restore /p:IsTestProject=true --filter "FullyQualifiedName~SvgServiceImageUriTests"
dotnet test tests/Svg.Skia.UnitTests/Svg.Skia.UnitTests.csproj -f net10.0 -c Release --no-restore /p:IsTestProject=true --filter "FullyQualifiedName~SKSvgTests.Load_RelativeImageHrefWithoutBaseUri_SkipsMissingImage"
dotnet build Svg.Skia.slnx -c Release --no-restore

Results:

• Model regression tests passed: 4 passed, 0 failed.
• SKSvg rendering regression passed: 1 passed, 0 failed.
• Release build passed with existing package advisory warnings.

Notes

• The PR summary file itself is intentionally not committed.
• The branch contains two commits:
  • Skip unresolved relative image loads
  • Cover missing relative image base URI