ArrowHelper works in multiple WebGL contexts

The closure ArrowHelper.js used re-used variables `lineGeometry` and `coneGeometry` in the constructor across multiple calls, and `axis` and `radians`.  This caused the error

    Error: WebGL: bindBuffer: object from different WebGL context (or older generation of this one) passed as argument

when using ArrowHelper in multiple WebGL contexts on the same page.  

BTW, ArrowHelper is generally a bit clunky, IMO.  That said, it may be too late to redefine arguments or methods..

And my diff is very simple, but github's diffing algorithm isn't the greatest, haha.