dpxenc: do not set coded_frame

Signed-off-by: Paul B Mahol <onemda@gmail.com>
dpxenc: simplifiy code using AVPixFmtDescriptor

Update rgb24 fate test as it no longer writes image in big endian
format.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
fate: increase coverage for dpx encoder

Signed-off-by: Paul B Mahol <onemda@gmail.com>
pngenc: Add 'dpi', 'dpm' options

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>