iOS: threaded WebGL on a CAEAGLLayer, deleteProgram fix

[triniwiz]

Summary

Runs iOS WebGL contexts on the shared WebGL thread, as #174 does on Android, by replacing the GLKView with a view backed by a CAEAGLLayer. Also fixes deleteProgram, which deleted a framebuffer instead of the program on every platform.

Threaded WebGL on iOS

• Why: WebGL drew through a GLKView. Its display, the per-frame present, only runs on the main thread, which is also the JS thread, so a heavy scene held the UI to the GPU's pace.
• Drawing buffer: the context now owns it. It is a framebuffer whose color buffer is stored in the view's layer (renderbufferStorage:fromDrawable:) and presented with presentRenderbuffer:. Both work from any thread. Depth/stencil is a packed renderbuffer taken from the context attributes. An offscreen context gets plain renderbuffers instead of a GLKView that is never shown.
• Threading: a context is threaded by default, as on Android; getContext(..., { threaded: false }) opts out. It is created, drawn, presented and dropped on the nsc-webgl thread, and requestAnimationFrame is held back while a canvas is behind (Canvas.threadedWebGL, NSCCanvas.threadedWebGL).
• Resize: reallocates the drawing buffer, queued behind earlier calls. A GL-backed 2D context (forceGL) reallocates its own as it resizes.
• On the context's thread: snapshots (toDataURL and friends), the texImage helpers, and video frame uploads. NSCRender gains drawFrame/drawFrameTexImage3D/drawFrameTexSubImage3D overloads that take the context, and makes its GL objects on first use there. @nativescript/canvas-media passes the context.
• Discard: depth and stencil are discarded after each present unless preserveDrawingBuffer is set, as GLKView did.
• Renames: CanvasGLKView becomes CanvasGLView, and NSCCanvas.getGlViewPtr() becomes getGlLayerPtr().

deleteProgram fix

• Both deleteProgram bindings (the slow path and the fast-API path) called canvas_native_webgl_delete_framebuffer with the program's name.
• So programs were never deleted, and a framebuffer sharing the program's name was deleted in its place.
• On iOS that can be the canvas's own framebuffer: three.js disposing its PMREM programs left the canvas drawing into nothing, shown as solid magenta.
• GLKView hid this by rebinding its framebuffer on every present. The C++ is shared, so Android leaked programs and lost framebuffers the same way.

Testing

• New spec: deleteProgram deletes the program and nothing else (WebGL 1 and 2). It fails without the fix (the program should be gone: expected false, got true).
• iOS simulator (iPhone 17 Pro): full spec 377/377. While WebGL runs, a sample shows the nsc-webgl thread busy.
• Starter app on the simulator: the home page, the raw WebGL shaders (WebGL 1, and WebGL 2 by swapping the context), three.js and the mixed dashboard render correctly.
  • three.js runs at 4 fps there, against 5 fps before this change: the simulator's GL is slow for that scene.
• Android (Galaxy A53), group by group: webgl 83/83, 2d 180/180, offscreen 37/37, imagebitmap 18/18, bitmaprenderer 15/15, canvassource 9/9.
• Not tested:
  • frame rates on a real iOS device;
  • video uploads into a threaded context;
  • a forceGL 2D canvas on iOS;
  • tvOS.

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