updates the internal vW property with the current viewport width in px
function updateMetrics() {

		isVwDirty = false;
		DPR = window.devicePixelRatio;
		cssCache = {};
		sizeLengthCache = {};

		pf.DPR = DPR || 1;

		units.width = Math.max(window.innerWidth || 0, docElem.clientWidth);
		units.height = Math.max(window.innerHeight || 0, docElem.clientHeight);

		units.vw = units.width / 100;
		units.vh = units.height / 100;

		evalId = [ units.height, units.width, DPR ].join("-");

		units.em = pf.getEmValue();
		units.rem = units.em;
	}