Improve flushing behaviour when there is a lot of space to fill (#2364).

Improve flushing behaviour when there is a lot of space to fill (#2364).

Previously a call to flush() could result in a lot of audio being
emitted from the decoder (if there is a big gap between the end
of the audio and the video).  This would end up being emitted in
one chunk from the player, crashing the audio analyser with an OOM
in some cases.