swscale/utils: factor (d + 1 < 4) out

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swscale/utils: remove useless ()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swscale/utils: check chroma width for fast bilinear scaler

Fixes artifacts where fast bilinear was used for downscaling chroma

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