png: allow encoding 16-bit grayscale

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  png: allow encoding 16-bit grayscale

Conflicts:
	libavcodec/pngenc.c

See: fdc013da7863190b9416d45830a9ad6d7fb854c2

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avformat/riff: treat msn audio like gsm_ms

Fixes decoding of 4 files from Ticket2854

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