Use device pixel ratio to calculate size of portal for max representation size
Created by: nickygerritsen
This uses window.devicePixelRatio
(if set) to calculate the element width and height.
This makes sure that devices that have a device-pixel-ratio of more than 1 (e.g. Retina iOS / Mac devices) will get the correct quality video. See also #1028 (closed) where I suggested doing this.