pnmdec: always output native pixel format

This simplifies the code

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/pnmdec: support more pnm files

Fixes Ticket1897

Some hunks that have become unneeded due to previous changes removed by the commiter.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>