Keep the webcam's aspect ratio while resizing the window

Lock window resizing to the aspect ratio of the webcam so that there are no black borders appearing. Just like it's advertised on elementary.io
https://elementary.io/images/screenshots/camera.jpg
On that screenshot you see no black borders on each side or on top and bottom of the window.
Even while resizing the window with the mouse, the window is locked to the canvas' aspect ratio of the webcam.
This feature also exists in audience (aka pantheon-videos) but it's broken and this feature exists in mpv-player.