feat(🤖): name the Android backing view explicitly

[wcandillon]

Why

Mirror of react-native-skia's change, so both canvases take the same Android options. opaque was a shortcut to get a SurfaceView on Android; on native it was consumed nowhere else, and on the web it only painted a white background. It moves under android as a compositing hint, and the backing view gets a name of its own.

The model

<Canvas />                                                     // TextureView, translucent
<Canvas android={{ opaque: true }} />                          // TextureView, no blend
<Canvas android={{ backingView: "SurfaceView" }} />            // compositor fast path, opaque by default
<Canvas android={{ backingView: "SurfaceView", opaque: false, zOrderOnTop: true }} />
<Canvas android={{ backingView: "HardwareBufferView" }} />     // opt-in, always composites with alpha

• android is a discriminated union. A TextureView takes opaque (false by default). A SurfaceView takes opaque (true by default: a translucent one blends against black below the window), zOrderOnTop and the new zOrderMediaOverlay, which composites it above the other SurfaceViews below the window; both z-orders apply in place on Android 11 and later and are read at mount before that. A HardwareBufferView takes no option, and falls back to a TextureView below Android 10 with a warning in development.
• Nothing derives the backing view from another prop any more: BackingViewKind holds the pure rules and Canvas.tsx mirrors them for the development warnings.
• On the web, alphaMode alone decides how the canvas composites, as in a browser; the white background is gone.
• The docs rewrite the Android rendering options and the transparency sections around this model, and say what a remount through a key costs rather than claiming Android cannot hand a canvas over (fix(🤖): keep a canvas on screen while it swaps its backing view in place #563 shows it can, at the price of per-release logic).

Migration

The top-level opaque prop is gone (TypeScript flags it) and android.surfaceType is android.backingView. The default moves from an opaque SurfaceView to a translucent TextureView: a full-screen scene that wants the compositor fast path asks for it with android={{ backingView: "SurfaceView" }}. Per the discussion, this lands as a plain feature commit, not a major.

Verification

• Pixel 8 (Android 17), Android View Props diagnostic: opaque applies in place on both views, the on-top and media overlay z-orders move a mounted SurfaceView without a remount, the hardware buffer view composites with alpha; no warning, error or crash in logcat. Screenshots of each state were checked.
• yarn tsc and yarn lint in the package, the example and the docs app (the docs app's two pre-existing TS2698 errors in TransparencyDemo.tsx are untouched), the Android build of the example, and the package's Jest suite against the Pixel 8 (44 suites, 222 tests).

[github-actions]

🎉 This PR is included in version 0.17.0 🎉

The release is available on:

• v0.17.0
• GitHub release

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