libswscale: add gray9 support

libavcodec/raw.c

libswscale/input.c

libswscale/swscale_unscaled.c

libswscale/utils.c

libswscale/version.h

tests/ref/fate/filter-pixdesc-gray9be

tests/ref/fate/filter-pixdesc-gray9le

tests/ref/fate/filter-pixfmts-copy

tests/ref/fate/filter-pixfmts-crop

tests/ref/fate/filter-pixfmts-field

tests/ref/fate/filter-pixfmts-fieldorder

tests/ref/fate/filter-pixfmts-hflip

tests/ref/fate/filter-pixfmts-il

tests/ref/fate/filter-pixfmts-null

tests/ref/fate/filter-pixfmts-pad

tests/ref/fate/filter-pixfmts-scale

tests/ref/fate/filter-pixfmts-vflip