In the start_frame() debug log, print the reference pos value rather than the evaluated value converted to int.
That's required because -1 is evaluated as NAN, which converted back
to int looks like a random number, this is especially annoying when
debugging sources with undefined pos (as the video4linux2 device).
Signed-off-by: Janne Grunau <firstname.lastname@example.org>
(cherry picked from commit 3e5bc7ff6a23014daf2eb0b9a22986753ae2ec4f)