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