src/vs/base/browser/browser.ts/WindowManager/getPixelRatio
class WindowManager:	public getPixelRatio(): number {
		let ctx: any = document.createElement('canvas').getContext('2d');
		let dpr = window.devicePixelRatio || 1;
		let bsr = ctx.webkitBackingStorePixelRatio ||
			ctx.mozBackingStorePixelRatio ||
			ctx.msBackingStorePixelRatio ||
			ctx.oBackingStorePixelRatio ||
			ctx.backingStorePixelRatio || 1;
		return dpr / bsr;
	}