- Enabled a single instance of decklink_consumer to manage a separate key output...

- Enabled a single instance of decklink_consumer to manage a separate key output driven by the ScheduledFrameCompleted callback of the fill playback instead of by its own callback. This seems to achieve perfect sync between key and fill with a decklink quad card. Enable by setting:
  <keyer>external_separate_device</keyer>
  the default is that the key device will be fill device + 1 but can be overridden via
  <key-device>[1..]</key-device>

- Removed synchronizing_consumer.