Thibault Saunier authored Jan 30, 2019 and Mathieu Duponchelle committed Jan 31, 2019

The way FlowCombiner combines the FLUSH doesn't work in the case
we have several "sinkpads" since any flush return FLUSH. But in the
case we have a seek where on one branch flush is done, we should
just say OK otherwise we might return FLUSHING to a src that has already
been seeked and is ready to process new buffers