decklink: Detect gaps on incoming stream times, issue warnings

decklink: Detect gaps on incoming stream times, issue warnings

When we receive a video or audio buffer, we calculate the next stream
time based on the current stream time + buffer duration. If the next
buffer's stream time is after that, we issue a warning.

This happens because the stream time incoming from Decklink should be
really constant and without gaps. If there is a gap, it means that
something went wrong, e.g. the internal buffer pool is empty (too many
buffers queued up downstream).

https://bugzilla.gnome.org/show_bug.cgi?id=781776