Changelog: add "H.261 RTP payload format (RFC 4587)"

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

This allows writing actual bitdepth in RGB48 when it isn't actually 16.

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