Avoid random black gaps between bits of content A and B when

Avoid random black gaps between bits of content A and B when
content B has had its start trimmed.  In this case when we seek
inaccurately in A we previously also seeked inaccurately in B.
This meant that at the transition the first frame we see of B
may not be the first frame of B.

After this we seek accurately in B so that we definitely get to the
start of it.

This is especially important if the start of B has been trimmed to
within two keyframes so that an inaccurate seek is not sufficient.