lavfi/overlay: show incoming frames on debug messages

This is especially useful to debug queue overflow issues.

Ideally we should be able to set the debug message at the
dualinput/framesync level, but they do not have the information related
to the filter context and the inlink, so cannot access much useful
information.

Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
