anyrender_skia: add opt-in Skia Graphite backend for Metal (macOS/iOS)

[nicoburns]

Summary

Adds an opt-in Skia Graphite backend to SkiaWindowRenderer on macOS/iOS (Metal). Ganesh stays the default everywhere, and Linux/Windows are unchanged (Ganesh via OpenGL).

# Graphite on Apple; Ganesh + GL elsewhere. Uses prebuilt Skia binaries on all platforms.
anyrender_skia = { version = "0.11", default-features = false, features = ["graphite"] }

Cargo features

• New ganesh (default) and graphite features.
• gl is now only enabled on non-Apple targets. The Metal backend never used it, and dropping it lets Apple builds match the published ganesh-…-metal-pdf / graphite-…-metal-pdf prebuilt binaries.
• graphite = ["dep:skia-safe"] activates an optional, Apple-only skia-safe entry that has the graphite feature. So on Linux/Windows (including CI's --all-features) graphite doesn't touch the Skia build.
• ganesh/vulkan use the weak skia-safe?/… syntax. Without it, skia-safe/ganesh would also activate that optional Graphite entry.
• rust-skia has no prebuilt binary with both Ganesh and Graphite. Enabling both still works, but Skia builds from source; in that case Graphite is used.

Resolved skia-bindings features, from cargo tree -e features:

target	default	--no-default-features --features graphite	--all-features
linux	ganesh gl pdf	ganesh gl pdf	ganesh gl pdf vulkan
windows	ganesh gl pdf textlayout	ganesh gl pdf textlayout	+ vulkan
macos/ios	ganesh metal pdf	graphite metal pdf	ganesh graphite metal pdf vulkan (source build)

Code

• metal.rs → metal/{ganesh,graphite}.rs. The CAMetalLayer setup is shared in metal::create_metal_layer.
• MetalGraphiteBackend on each frame:
  • prepare: wraps the drawable's texture via graphite::mtl::backend_textures::make_metal and graphite::surfaces::wrap_backend_texture.
  • flush: recorder.snap() → context.insert_recording → context.submit, then presents on the same MTLCommandQueue.
• Graphite's default ImageProvider drops raster images: they aren't drawn, and Skia logs Couldn't convert SkImage to a Graphite-backed representation. skia-safe doesn't expose custom image providers, so:
  • SkiaScenePainter gets an optional &mut graphite::Recorder. The window renderer gets it from the new SkiaBackend::recorder().
  • Image brushes are uploaded with graphite::images::texture_from_image before the image shader is built. The result goes into the existing image_shader cache.
  • shader_from_image_brush is split into image_from_image_data + shader_from_image for this.
• New public SkiaScenePainter::new_graphite(canvas, cache, recorder) for painting into your own Graphite surfaces, e.g. offscreen. It's used by the Blitz benchmark in [Testing] anyrender_skia Graphite backend (anyrender#103) + skia_paint_bench blitz#983.

Known gaps

• Uploaded textures have no mipmaps: skia-safe's texture_from_image doesn't take RequiredProperties. With ImageQuality::Medium/High, Skia falls back to linear, non-mipmapped sampling for these images.
• Vulkan Graphite (Linux/Windows) isn't included. skia-safe has no binding to wrap a VkImage as a Graphite BackendTexture.

Performance

• Offscreen Blitz page benchmarks are in [Testing] anyrender_skia Graphite backend (anyrender#103) + skia_paint_bench blitz#983. They ran on a macOS VM, so the GPU numbers aren't representative of real hardware.
• On those runs, Graphite spends about 1.5x longer recording draw calls on 3 of the 4 pages. Total frame time ranges from 3% faster to 22% slower than Ganesh.

Testing (macOS arm64)

• bunnymark release builds with each engine; both render correctly. Blitz rdme also renders correctly on Graphite (see blitz#983). Graphite logged no warnings.
• cargo clippy -D warnings for the workspace (default features) and for anyrender_skia with --no-default-features --features graphite (with and without pixels_window_renderer). cargo doc with Graphite.

Graphite (1000 bunnies):

Ganesh (default, 500 bunnies), for comparison:

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/7b8d8492640e46d08ea6cd8b5ab63dbb
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/7b8d8492640e46d08ea6cd8b5ab63dbb?variant=devin-insiders
Requested by: @nicoburns

[staging-devin-ai-integration]

I'll fix CI failures and address comments from users with write access that start with 'Devin'.

• Disable automatic comment, CI, and merge conflict monitoring