One of the causes of this bug is that the h264 parser defaults low_delay
to 1, but the h264 codec defaults low_delay to 0. Really Ugly.

After many hours of looking at this, I'm still not sure how has_b_frames
is *intended* to behave, but to me the implementation appears way more
complicated than it ought to be.

My patch relies on the encoder to set an optional field in the SPS. This
works for libx264 streams, but I'm not sure that all h264 encoders will
set it.