The frame_centre_horizontal/vertical_offset values contained in picture
display extensions are actually signed values (i.e. it is possible to
indicate that the display device should add black bars/pillars).

The files sony-ct3.bs and tcela-6.bits (which are both used in fate
tests for mpeg2_metadata) contain picture display extensions; the former
even contains a negative frame_centre_vertical_offset. Fortunately, the
old code did not damage the picture display extensions when one did a
cycle of reading and writing. For the same reason the fate tests needn't
be updated either.

Furthermore these fields now use the trace output for matrices.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>