Don't allow the queue to get too big with REPEAT frames otherwise

there's a long delay at the end of the job while they are written.
We must still write FULL frames even if the queue is long (we only
stop doing that if the queue has too many FULL frames i.e. too much
memory consumption).  With this commit we stop writing REPEAT/FAKE
frames when the queue gets long and assume there will always be a
sequence image for writing and hence the main writer thread will reduce
the queue given time.