icon-halo-width unit update

@ansis just want to clarify on this -- I know text-halos max out at 1/4 of the text size. Do icons behave the same? I'm asking because I'm just not sure how [this](https://github.com/mapbox/mapbox-gl-js/blob/dev-pages/js/render/drawsymbol.js#L110) should affect them...