Add a `bind_external_framebuffer` function to the web sys context

[expenses]

This is necessary in order to bind the external framebuffer that you get from WebXR: https://developer.mozilla.org/en-US/docs/Web/API/XRWebGLLayer/framebuffer

I'm using this for my branch of wgpu that can render to WebXR framebuffers: https://github.com/gfx-rs/wgpu/compare/master...expenses:webxr

[expenses]

bind_external_framebuffer is probably a better term, actually.

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[grovesNL]

bors r+

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