Allow multiple sources in a viewport to change what portion they take up

[faultyserver]

We had multiple runs at SGDQ 2023 that had handcams alongside the main runner cameras. We also had a special layout that had a normal runner camera frame, but also a three-head shot of additional commentators.

With the current setup, we added two camera sources to a single viewport, then stacked them to fill the space. But viewport-assign will always cause each source to take up 1/nth of the viewport space, and that can't be adjusted. That means that things like handcams are shown at an equal size to the runner camera, when a preferable layout would have the handcam be a short and wide viewport with the runner camera on top being more square and taking up more of the relative space.