Improve source handling

[dexterlb]

• Report to sources whether they are visible or invisible, so they can decide to not do work
  • Pause external ffmpeg cli by SIGSTOP (only when user wants to, via config option for the source)
  • If gst sinks and sources #25 gets implemented, pause/play gst
• Do not send frames to the GPU when there's no need to (Optimise frame pushing #3)
• Properly manage external command lifecycle (ffmpeg processes are not always killed #16)

[MartijnBraam]

Downside of making sources stop working is that you get inconsistent performance (and possibly glitching because a source was filling the buffer while you switch to it)

Also with a multiview sink all the sources would be active anyway.

[dexterlb]

The usecase I'm imagining is only for sources that play videos, e.g. if you want to play an animation before switching to a talk