decklinkvideo/audiosrc: Add GstReferenceTimestampMeta with the stream time to each buffer

This is basically a frame counter provided by the driver and it's
advancing at the speed of the HDMI/SDI input. Having this available on
each buffer allows to know what constant-framerate-based timestamp each
frame is corresponding to and can be used e.g. to write out files
accordingly without having the local pipeline clock timestamps used.

https://bugzilla.gnome.org/show_bug.cgi?id=779213
2 files changed