WebXR - expose framebuffer scale factor

[Maksims]

It is possible to control the resolution of the XR framebuffer by providing a scale factor when starting an XR session.

New APIs:

// pc.XrManager
xr.start(camera, type, space, {
    framebufferScaleFactor: 1.0 // optional property by which framebuffer resolution will be multiplayed when session is being created
});
xr.framebufferScaleFactor // read-only value of a current session framebuffer scale factor

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[mvaligursky]

Nice one.