The new new decode API requires the decoder to ask for the next input
packet, and it cannot just return EAGAIN if that packet cannot be
processed yet. This means we must finally confront how we get this
decoder to block when the input buffer is full and no output frames
are ready yet.

In the end, that isn't too hard to achieve - the main trick seems to
be that you have to aggressively poll the hardware - it doesn't seem
to make any forward progress if you sleep.
Signed-off-by: James Almer <jamrial@gmail.com>