decklinksrc: Stop using the "hardware" timestamps and directly use the pipeline clock

decklinksrc: Stop using the "hardware" timestamps and directly use the pipeline clock

The hardware timestamps have no relation to when frames were produced,
only when frames arrived somewhere in the hardware. Especially there is
no guarantee that audio and video will have the same hardware timestamps
although they belong together, and even more important: the rate with
which the hardware timestamps increase is completely unrelated to the
rate with which the frames are captured!

As such we can as well use the pipeline clock directly and stop doing
complicated calculations. Also as a side effect this allows now running
without any pipeline clock, by directly making use of the stream times
as reported by the driver.

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