decklinksrc: Keep track of a window of capture/stream time mappings and calculate...

decklinksrc: Keep track of a window of capture/stream time mappings and calculate relationship

Based on this we calculate the actual capture time, which should get us
rid of any capturing jitter by averaging it out.

Also add a output-stream-time property which forces the elements to
output the stream time directly instead of doing any conversion to the
pipeline clock. Use with care.

https://bugzilla.gnome.org/show_bug.cgi?id=774850