Support multiple M/Es

[jamesremuscat]

• Configure which M/E to control as "primary" (perhaps not runtime-configurable - that would just be an opportunity for people to break things) - we want to be able to control M/E 2 primarily
• Store and display tally information for alternative M/Es
  • Idea: use a red bottom border for sources live in other M/Es but not our primary one (green for preview); full red border for live in our primary M/E
  • Generate synthetic tally for ATEM M/E 2 jamesremuscat/avx#81 is a prerequisite since the ATEM only generates tally for M/E 1

[jamesremuscat]

I think we'll also have to swap from using a downstream keyer for overlaying Visuals PC onto the background to an upstream one, since I think that both DSKs appear on the M/E 1 output.

I could be wrong since I have a feeling that in the version of the ATEM Control software we're using, one of the DSKs is shown on the M/E 1 page and one on the M/E 2 page, but that might have been an error in that version of the software.

[jamesremuscat]

I've started work on this at https://github.com/jamesremuscat/av-control/tree/feature/atem-multi-me.