That demo is still the right way to go. Just instead of having a dropdown list of sources, you can automatically look through the list and find a source with the name "back" in it somewhere, when the user clicks "get camera". Although note that for security reasons you can't access the names of the camera devices until the user has approved camera input previously on that site... so you can only do the automatic picking on subsequent visits. I think given that restriction, it probably is easiest just to show the user a dropdown...