Fixes multiple seeking issues. When doing ACCURATE or normal
non-KEYUNIT seeks, mxfdemux would just send data from the
edit unit that covered the seek position, whether that's
a keyframe or not. Decoders would only output things from
the next keyframe then, which means there's a gap between
the start of the segment and the first decoded data in
some cases. In combination with gst-editing-services this
might result in a frozen picture for the duration of that
gap at the beginning (if videorate fixes up the first
buffer's start timestamp to cover the entire gap), or
a black frame (if no videorate is used and videomixer
fills the gap). Also fixes A/V sync issue when requesting
a KEYUNIT seek.
4898a51afe