avcodec/utils: support non edge emu for grayscale

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/snow: add gray/alpha/gbr

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/snow: gray support

Fixes Ticket839

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