Moved bound callbacks in pose sensor into variables and added a stop function to turn off event listeners.
Moved bound callbacks in pose sensor into variables and added a stop function to turn off event listeners.

bkkl/bkkl.github.io,marciot/webvr-polyfill,immersive-web/webvr-polyfill,borismus/webvr-polyfill,marciot/webvr-polyfill,jsantell/webvr-polyfill,bkkl/bkkl.github.io,jsantell/webvr-polyfill,immersive-web/webvr-polyfill,borismus/webvr-polyfill