Add screen cast layout parameters
Screen casting, including screen sharing and remote desktop, currently have a couple of properties communicated via one-shot options sent when starting a screen cast via the portal. It'd be better if these were part of the stream parameters. There are two benefits to this:
- Properties can change; e.g. a screen shared monitor position can change without the stream being recreated
- In some cases, notably remote desktop to a headless session, it should be possible to affect the position and logical size of a screen cast, effectively being a virtual monitor, in the global compositor coordinate space, to e.g. match the monitor layout of the remote desktop client.