avfilter/vf_transpose: Fix regression with packed pixel formats

      Regression since: c6939f65a116b1ffed345d29d8621ee4ffb32235
Found-by: Paul B Mahol <onemda@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>