It was never really necessary to update the source capabilities before
making sending a Request message. It was convenient though, since we
didn't store the previous Source_Capabilities message. Now we do, so
it's possible to make a new Request based on that.
This commit makes the Sink do just that. It doesn't follow the spec's
state diagram for this, but it's a minor change that works better with
this firmware's design, so I don't mind. There's a comment explaining
that too, so future folks won't be confused by it either.