Implement initial test runner / CI build test

[drasticactions]

This adds an initial test runner and CI test workflow for NImpeller

• It runs on llvmpipe (or theoretically another software GL instance). This is so it can be consistent (since it should always be the same running through that than whatever hardware GL you happen to have) and because running this against Nvidia EGL on my machine would segfault on some of the tests in its drivers. That requires further investigation on my part, but it's not Impeller/NImpeller-related AFAIK.
• It's broken down into two parts: Binding tests that exercise the library and don't render UI, and "Golden" tests (Flutter's term) that render out to PNGs that are rendered out as part of the first run, then stored and verified against for the future. They were based on their unit tests, the idea being that as we increase our binding support, we can match what they render to, since there shouldn't be any difference between using our bindings and using their engine directly.

[Copilot]

Pull request overview

Adds an initial automated test harness for NImpeller, including a headless software-GL renderer for deterministic golden-image regression tests, and wires it into GitHub Actions CI on Ubuntu.

Changes:

• Introduces a new tests/NImpeller.Tests xUnit v3 test project with unit tests plus headless GL-based rendering/golden tests.
• Adds a deterministic “golden scenes” suite and baseline image storage/compare utilities.
• Adds a CI workflow to install Mesa/llvmpipe + SDL deps, generate bindings, and run tests under Xvfb.

Reviewed changes

Copilot reviewed 20 out of 44 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/NImpeller.Tests/Unit/StructHelperTests.cs	Adds unit tests for struct helpers and conversions.
tests/NImpeller.Tests/Unit/PathBuilderTests.cs	Adds unit tests for path builder APIs.
tests/NImpeller.Tests/Unit/LifetimeTests.cs	Adds lifetime/idempotent dispose tests for wrappers.
tests/NImpeller.Tests/Unit/BuilderStateTests.cs	Adds tests for builder save/restore and display list creation.
tests/NImpeller.Tests/TextureInteropTests.cs	Adds a GL texture interop golden test.
tests/NImpeller.Tests/test.runsettings	Forces software GL (llvmpipe) via environment variables for test runs.
tests/NImpeller.Tests/Scenes/IScene.cs	Defines scene interface and render parameters for golden tests.
tests/NImpeller.Tests/Scenes/GoldenScenes.cs	Implements deterministic scenes used for golden-image regression.
tests/NImpeller.Tests/RenderGate.cs	Gates render tests based on availability and CI/local behavior.
tests/NImpeller.Tests/NImpeller.Tests.csproj	Adds the new test project and dependencies (xUnit v3, Silk.NET, runsettings).
tests/NImpeller.Tests/Impeller.targets	Copies the correct native Impeller library into test output.
tests/NImpeller.Tests/Headless/RawImage.cs	Implements raw image storage/comparison and PNG output for diagnostics.
tests/NImpeller.Tests/Headless/ImpellerGLFixture.cs	Provides a shared GL fixture for tests and scene rendering.
tests/NImpeller.Tests/Headless/HeadlessImpellerContext.cs	Implements a thread-affine headless SDL+GLES context, offscreen rendering, and readback.
tests/NImpeller.Tests/Goldens/.gitkeep	Documents committed golden baseline storage and update flow.
tests/NImpeller.Tests/GoldenImageTests.cs	Implements the golden-image test runner and baseline update logic.
tests/NImpeller.Tests/AssemblyInfo.cs	Disables test parallelization due to shared thread-affine GL context.
README.md	Documents how to run the test suite locally.
NImpeller.sln	Adds the test project to the solution.
.github/workflows/tests.yml	Adds Ubuntu CI job to install deps, generate bindings, and run tests under Xvfb.

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