Fixes #373 issues on WebXR with Oculus Browser

[fernandojsg]

This PR address several issues with the current implementation:

• Bind the session fb when presenting
• Force setting the canvas size to the session fb size
• Expose InternalBrowser from emscripten wasm module
• Hook createContext from WebGL to force xrCompatible, instead of using ctx.makeXRCompatible()
• Use local-floor instead of local when requesting session
• Add framerate stats
• Hook Unity's RequestAnimationFrame to be called as when entering VR the window.rAF is not called anymore.
• Add resize method and add a listener to the resize event.

Kudos to @De-Panther as many of the changes were based on his fork and hints, thanks for that!

Tested on Unity 2019.3.4f1 and Oculus Quest

Fixes #373