decklink: calculate the decklink output time from the internal clock

Matthew Waters requested to merge ystreet/gst-plugins-bad:decklink-timing into master

Fixes the time calculations when dealing with a slaved clock (as will occur with more than one decklink video sink), when performing flushing seeks causing stalls in the output timeline, pausing.

Tighten up the calculations by relying solely on the internal time (from the internal clock) for determining when to schedule display frames instead attempting to track pause lengths from the external clock and converting to internal time. This results in a much easier offset calculation for choosing the output time and ensures that the clock is always advancing when we need it to.

This is fixup to the 'monotonically increasing output timestamps' goal in: bf849e9a

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