swscale: add gbr(a)p16 output support

libswscale/output.c

libswscale/utils.c

tests/ref/fate/filter-pixdesc-gbrap16be

tests/ref/fate/filter-pixdesc-gbrap16le

tests/ref/fate/filter-pixdesc-gbrp16be

tests/ref/fate/filter-pixdesc-gbrp16le

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-lut

tests/ref/fate/filter-pixfmts-null

tests/ref/fate/filter-pixfmts-pad

tests/ref/fate/filter-pixfmts-scale

tests/ref/fate/filter-pixfmts-vflip