Many changes to avpinput_decklink

Most of these were likely done live at a production with the mobile
encoder, thus the whopping commit. Sorry.
DeckLink input: keep audio and video locked

When a video frame is lost, deliberately lose an audio frame.
This prevents any sync issues due to dropped video frames.
BlockSet: print which block wasn't found

This makes debugging easier.
avpinput_decklink: add audio mono-mix filter

Reworked FilterChain to be a template, and added an audio filter
chain. Passing '-m' on the command line will result in an output
where all channels are a mono mixdown of input channels.