Dimensions / pixel formats for scaling must be set through the -s / pix_fmt options
or the scale / format filters. Otherwise there are mismatches between whet is
in/output to the scaler and for what the scaler is configured

Fixes Ticket4856
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a0af9fd9)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>