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 not designed well, 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...