lavf/mxfdec: add -field_rate option

Interpret sample rate as field rate for j2k codec with separate fields
or segmented frame layout.

This option is required to obtain a correct frame since S422M specifies
that the track sample rate could either be the frame rate or the field
rate for separate field layout.

For segmented frame layout, the track sample rate is supposed to be the
frame rate however some sample seems to use it as the field rate.