OffscreenCanvas

[triniwiz]

Summary

Adds OffscreenCanvas, plus fixes found while testing it on iOS and Android.

OffscreenCanvas

• A canvas with no view. It draws through the same native contexts as Canvas: on a detached canvas made at the first getContext(), or, after canvas.transferControlToOffscreen(), on that canvas's own surface so its frames show in the view.
• Contexts: 2d, bitmaprenderer, webgl, webgl2, webgpu.
• transferToImageBitmap() hands over the frame and clears the canvas, as a present would. convertToBlob() encodes it. contextlost/contextrestored are forwarded from the host canvas.
• Canvas: getContext() moves to CanvasBase, which throws InvalidStateError once the canvas is transferred and otherwise calls the host's _getContext(). transferControlToOffscreen() refuses a canvas that has a context or was already transferred. A transferred canvas's width/height setters throw, and its OffscreenCanvas resizes it through _resizeBitmap().
• Image sources: drawImage, createPattern, createImageBitmap, texImage2D/texSubImage2D and copyExternalImageToTexture take an OffscreenCanvas where they take a Canvas. isOffscreenCanvas checks the tag, so these modules don't need to import it.
• Exports: OffscreenCanvas and OffscreenCanvasRenderingContext2D are exported, set on global, and on the polyfill's window. The Canvas typings gain transferControlToOffscreen() and threadedWebGL.
• It still can't move to a Worker: the contexts belong to the UI thread.

Fixes

• V8 string helpers read freed memory. ConvertFromV8StringView/ConvertFromV8StringViewValue returned a std::string_view into a v8::String::Utf8Value destroyed on return. On iOS, __base64Decode/__base64Encode returned '' for any input, and a WebGPU featureLevel option was ignored. They now return std::string.
• getParameter returns what was bound. FRAMEBUFFER_BINDING (and READ_FRAMEBUFFER_BINDING, PIXEL_PACK_BUFFER_BINDING) came back as a wrapper around the bare GL name, which binds nothing when passed back, and on iOS named the view's framebuffer when none was bound. So the usual save → bind → restore pattern left the drawing buffer bound. The context now keeps the objects it bound, clears them when deleted, and answers without a native round trip.
• iOS WebGL canvas as an image source. canvas_native_webgl_read_drawing_buffer bound framebuffer 0, but on iOS the drawing buffer is the GLKView's framebuffer. texImage2D/copyExternalImageToTexture from a WebGL canvas read back transparent there.
• canvas_native.h regenerated.

Testing

• New spec group offscreen in tools/demo/canvas/spec/offscreen.ts (37 tests: API rules, each context, image-source use, transfer), run with --demo=canvas-spec --suite=offscreen. Also 4 WebGL tests for the binding fix.
• iOS simulator (iPhone 17 Pro): full spec 375/375.
• Android (Galaxy A53), group by group: offscreen 37/37, 2d 180/180, webgl 81/81, imagebitmap 18/18, bitmaprenderer 15/15, canvassource 9/9.
  • webgpu.video (2), scaling and lifecycle fail on origin/master too; scaling and lifecycle also vary from run to run.
  • Running every group in one Android process gets killed by lmkd on origin/master too. Native heap grows about 7 MB per WebGL test until GC runs.
• Not tested: a transferred canvas on screen (the tests use detached canvases), and Windows.

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