Fixes outputted frame sequence when performing a seek
i.e. when seeking backwards, the first frame after the seek was a frame
from the future. This would result in GstVideoDecoder essentially
marking all the timestamps as essentially bogus and the base class would
attempt to compensate. A visible indication of this was 'decreasing timestamp'
warning after a seek.
https://bugzilla.gnome.org/show_bug.cgi?id=790478
4e422bec4f