Run render tests against llvmpipe/lavapipe MESA rasterizers

[kekekeks]

With this PR the tests are run against MESA's llvmpipe/lavapipe rasterizers compiled into a standalone library (not the system one, so results should be more or less reproducible).

There are some changes in vulkan/opengl support libs to make it to work:

1. optional support window-less egl context
2. optional support for swapchain-less/surface-less vulkan instances/devices.

Direct readback from GPU API is used to make sure that we don't need to expose/use backend-provided snapshot mechanisms.

It somewhat uncovered gpu state teardown problems that will be re-addressed later (see vulkan dispose diff + gpu teardown code in test helper).

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0067502-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[Copilot]

Pull request overview

This PR makes the Skia render test suite runnable against deterministic MESA software rasterizers (llvmpipe for GL / lavapipe for Vulkan) by introducing offscreen GPU render paths with direct readback, and by loosening platform requirements around window/surface/swapchain support for those offscreen scenarios.

Changes:

• Add a bundled MESA software-renderer dependency and wire render tests to optionally run additional GL/Vulkan GPU passes.
• Add offscreen GL (FBO) and Vulkan (VkImage) render-target surfaces that read pixels back into a writable bitmap for image comparison.
• Extend EGL and Vulkan initialization/device creation to support surfaceless EGL and surface/swapchain-less Vulkan instances/devices.

Reviewed changes

Copilot reviewed 30 out of 30 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/Avalonia.Skia.RenderTests/Avalonia.Skia.RenderTests.csproj	Adds the MESA software-renderer NuGet dependency to the render test project.
tests/Avalonia.RenderTests/TestRenderHelper.cs	Refactors render helper and adds GPU readback-based composited rendering + backend teardown.
tests/Avalonia.RenderTests/TestBase.cs	Produces and compares additional Mesa GL/Vulkan outputs with optional error tolerance/skip controls.
tests/Avalonia.RenderTests/Shapes/PathTests.cs	Relaxes allowed error for GPU outputs on a widened-path case.
tests/Avalonia.RenderTests/Shapes/EllipseTests.cs	Relaxes allowed error for GPU outputs on aliased circle rendering.
tests/Avalonia.RenderTests/Mesa/VulkanReadbackPlatformSurface.cs	New offscreen Vulkan render target that copies VkImage contents to CPU for comparisons.
tests/Avalonia.RenderTests/Mesa/MesaSoftwareRenderer.cs	New helper to create llvmpipe/lavapipe-backed platform graphics from the bundled native library.
tests/Avalonia.RenderTests/Mesa/FboReadbackGlPlatformSurface.cs	New offscreen GL render target that reads FBO pixels back to CPU for comparisons.
tests/Avalonia.RenderTests/Media/VisualBrushTests.cs	Relaxes allowed error for GPU outputs on a DPI-sensitive visual brush case.
tests/Avalonia.RenderTests/Media/DrawingContentTests.cs	Skips Mesa GPU comparisons for custom-flow change rendering cases.
tests/Avalonia.RenderTests/Controls/CommandBarTests.cs	Adds a slightly higher allowed error for GPU outputs for a themed command bar case.
tests/Avalonia.RenderTests/Controls/AdornerTests.cs	Prevents duplicate adorner insertion across repeated attach/detach cycles during composited renders.
tests/Avalonia.RenderTests/Composition/DirectFbCompositionTests.cs	Skips Mesa GPU comparisons for direct-framebuffer composition tests.
src/Skia/Avalonia.Skia/Gpu/Vulkan/VulkanSkiaGpu.cs	Improves Vulkan Skia GPU disposal behavior and recognizes swapchain-less Vulkan render target surfaces.
src/Avalonia.Vulkan/VulkanOptions.cs	Adds options to allow creating Vulkan instances/devices without surface/swapchain extensions.
src/Avalonia.Vulkan/VulkanContext.cs	Supports creating a render target directly from an IVulkanRenderTargetPlatformSurface.
src/Avalonia.Vulkan/UnmanagedInterop/VulkanStructs.cs	Adds Vulkan structs needed for image-to-buffer copy readback support.
src/Avalonia.Vulkan/UnmanagedInterop/VulkanInstanceApi.cs	Marks surface-related instance procs as optional to support offscreen-only instances.
src/Avalonia.Vulkan/UnmanagedInterop/VulkanEnums.cs	Adds VkBufferUsageFlags needed for readback buffer creation.
src/Avalonia.Vulkan/UnmanagedInterop/VulkanDeviceApi.cs	Adds buffer/mapping/copy APIs and marks swapchain-related procs as optional.
src/Avalonia.Vulkan/IVulkanPlatformSurface.cs	Introduces IVulkanRenderTargetPlatformSurface for swapchain-less Vulkan rendering surfaces.
src/Avalonia.Vulkan/Interop/VulkanInstance.cs	Makes VK_KHR_surface enabling configurable via options.
src/Avalonia.Vulkan/Interop/VulkanDevice.Create.cs	Makes VK_KHR_swapchain enabling configurable via options.
src/Avalonia.Vulkan/Avalonia.Vulkan.csproj	Grants test assembly access to Vulkan internals used by the new readback surface.
src/Avalonia.OpenGL/GlInterface.cs	Adds glReadPixels binding needed for FBO readback.
src/Avalonia.OpenGL/Egl/EglDisplayUtils.cs	Allows probing EGL configs that only support PBuffer surfaces when requested.
src/Avalonia.OpenGL/Egl/EglDisplayOptions.cs	Adds an option to allow PBuffer-only EGL configs (surfaceless EGL scenarios).
src/Avalonia.OpenGL/Egl/EglDisplay.cs	Threads through the new EGL option to config selection.
src/Avalonia.OpenGL/Egl/EglConsts.cs	Adds EGL_PLATFORM_SURFACELESS_MESA constant.
Directory.Packages.props	Pins the unofficial.mesa.softwarerenderer package version.

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0067505-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

LGTM!