Dedicated speaker view/mode during conference calls

### Your use case

#### What would you like to do?
Allow an account/user to maintain focus during conference calls, or be selected by another account/user as the video they wish to stay in focus.

#### Why would you like to do it?
I tried to use Matrix to stream a board game to a friend. However, we could find no way to lock focus to the board game camera. It would keep flipping to whoever spoke last. In the end, we had to switch back to Discord, which has such a feature.

#### How would you like to achieve it?
Hoping in the direction of element-hq.

### Have you considered any alternatives?

No.

### Additional context

This may also be useful for actual conference calls in which there is a single speaker for the majority/entirety of the call.