concat: Reset segment base offset after FLUSH_STOP with reset_time = TRUE
concat: Reset segment base offset after FLUSH_STOP with reset_time = TRUE

If the reset_time value of a FLUSH_STOP event is set to TRUE, the pipeline
will have the base_time of its elements reset. This means that the concat
element's current_start_offset has to be reset to 0, since it was
calculated with the old base-time in mind.

Only FLUSH_STOP events coming from the active pad are looked at.

Signed-off-by: Carlos Rafael Giani <d7d2e6b3863b2910d2ca34828bc651c85d88df6f@pseudoterminal.org>

krichter722/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,centricular/gstreamer,krichter722/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,jpxiong/gstreamer