Generate synthetic tally for ATEM M/E 2

[jamesremuscat]

The ATEM only provides tally information for M/E 1.

It would be useful to be able to provide tally information for M/E 2 also, so that the two M/Es can be used semi-independently to drive two vision mixes. We can generate this from looking at the current program/preview sources plus upstream and downstream keyers.

(Hacky workaround is to configure an upstream keyer on M/E 1 that uses M/E 2 as a fill and black as a key source, and leave it permanently active; the ATEM will then consider sources on both M/Es live. However, this loses information about which source is live on which M/E.)