Re-add Util.getQuaternionAngle to fix debug mode

[MattiasBuelens]

When debug mode is activated on mobile devices, the polyfill throws a lot of errors:

Uncaught TypeError: Util.getQuaternionAngle is not a function
    at ComplementaryFilter.run_ (webvr-polyfill.js:4237)
    at ComplementaryFilter.addGyroMeasurement (webvr-polyfill.js:4190)
    at FusionPoseSensor.updateDeviceMotion_ (webvr-polyfill.js:4039)
    at FusionPoseSensor.onDeviceMotion_ (webvr-polyfill.js:4009)

Steps to reproduce:

1. Open http://googlevr.github.io/webvr-polyfill/examples/basic/?debug=true in Chrome on Android
2. Attach a remote Chrome debugger
3. Click the VR button

It appears that Util.getQuaternionAngle was (accidentally) removed some time after v0.2.6, but it was still used in debug mode. This PR re-adds this missing function.

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[MattiasBuelens]

@googlebot I signed it

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[MattiasBuelens]

Perhaps off-topic, but what's the value of this debug mode? We accidentally ran into this because a customer was using ?debug for debugging their own website, not the WebVR polyfill. If it's been broken for so long and nobody's actually using it, maybe it's better to remove it altogether?

[jsantell]

Thanks for reporting this! It seems debug is somewhat vestigial, not one I ever used or dove into since I've started maintaining several months ago.

Long term, I think I'd rather remove this, or make it a value in the configuration, rather than a query param. In the mean time, let's merge this util back in. Thanks for digging into this!