swscale/output: Fix rounding offset in yuv2gbrp_full_X_c()

libswscale/output.c

tests/ref/fate/filter-pixdesc-gbrp10be

tests/ref/fate/filter-pixdesc-gbrp10le

tests/ref/fate/filter-pixdesc-gbrp12be

tests/ref/fate/filter-pixdesc-gbrp12le

tests/ref/fate/filter-pixdesc-gbrp14be

tests/ref/fate/filter-pixdesc-gbrp14le

tests/ref/fate/filter-pixdesc-gbrp9be

tests/ref/fate/filter-pixdesc-gbrp9le

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

tests/ref/lavf/dpx