encodebin: Forward upstream force-key-unit events to encoder
streamsplitter: Drop duplicated force-key-unit events
Forward force-key-unit event only once for the corresponding sequence number.
streamcombiner: Forward upstream force-key-unit events to all sinkpads
streamcombiner element forwards a upstream event only to one sinkpad.
When the streamcombiner is used with encodebin, the sinkpad
corresponding to pass-through path is configured before that of encoder,
and therefore streamcombiner forwards upstream events only to
the firstly configured one (i.e., pass-through path).
Fixes: #429 (closed)