Fix pointerlock-test

The pointerlock-test was only true if you had already locked the pointer when Modernizr runs: `document.pointerLockElement` is `null` when not in pointerlock, which when cast to a bool becomes false.

Rewritten to use `exitPointerLock`, quite similar to how the fullscreen-test works.

Checked in MacOS: 
* Chrome (true) 
* Firefox (true) 
* Safari (false) 
* Opera (false)

as expected per [MDN](https://developer.mozilla.org/en-US/docs/API/Pointer_Lock_API).