ffmpeg: fix reinitializing with threads while flushing

With threads the decoder has a delay and will thus have multiple
frames at EOF left in its buffers which will be returned when flushing
the decoder. The code that extracts such frames from the decoder at the
end does not pull frames from the filtergraph, thus when one of these
frames causes the filtergraph to be reinited, the frames still inside
the graph at that point re lost

This commit changes the flushing to be more similar to normal decoding
and 1 frame at a time

Fixes hqx fate with threads

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>