fix(canvas): Android 2D presents nothing after a resize

[triniwiz]

Since #165, every 2D canvas on Android stays blank once it has been resized, which is all of them after they are sized to their layout. This happens with the render thread on or off (Canvas.threaded2D = false).

Cause. On Android, 2D now draws into an offscreen Skia render target and copies it to the window when it presents. resize_gl (canvas-c) chose the window's framebuffer by reading GL_FRAMEBUFFER_BINDING right after flushing Skia. At that point the binding is Skia's offscreen framebuffer, not the window's (0). So the re-created window surface wrapped the offscreen framebuffer, and every present copied it onto itself. On-device logging showed bindings of 2, 3 and 10–12 at each resize.

Changes

• crates/canvas-2d/src/context/surface_gl.rs: new Context::window_framebuffer(), which returns the framebuffer the existing window surface wraps, read from its backend render target.
• crates/canvas-c/src/c2d/context.rs: resize_gl uses that framebuffer when 2D presents through a window. It still reads the GL binding otherwise (pbuffers, other platforms), so their behaviour is unchanged.

Testing

Android, Galaxy A53: in the starter app, the home hero, flow field, clock and live chart canvases all render, with the default threaded path and with threaded2D = false. Before the change they were black (pure 0,0,0, the empty surface).

Not tested: iOS and Windows are not affected by the offscreen path. Binaries under platforms/ are not rebuilt in this PR.

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