swscale: Do not loose precission on yuv values after rgb->yuv.

libswscale/swscale.c

libswscale/swscale_template.c

libswscale/utils.c

libswscale/x86/swscale_template.c

libswscale/x86/swscale_template.h

tests/ref/lavf/pixfmt

tests/ref/lavfi/pixfmts_scale_le

tests/ref/vsynth1/flashsv

tests/ref/vsynth1/jpegls

tests/ref/vsynth1/msvideo1

tests/ref/vsynth1/qtrle

tests/ref/vsynth1/rgb

tests/ref/vsynth2/flashsv

tests/ref/vsynth2/jpegls

tests/ref/vsynth2/msvideo1

tests/ref/vsynth2/qtrle

tests/ref/vsynth2/rgb