dpx: always write the image offset

According to the DPX file format description found at
http://www.fileformat.info/format/dpx/egff.htm

 the ImageElement part of
the GenericImageHeader also contains an an offset to the real image data
beside the same member that can be found in the GenericFileHeader.

Libav keeps this member empty (=0) while some applications expects it to
be filled properly. FATE test updated accordingly.

Bug-Id: 742
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>