avcdoec/huffyuvenc: optimize sub_left_prediction()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Move add_hfyu_left_prediction_int16 to losslessviddsp

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Move add_hfyu_median_prediction_int16() to losslessviddsp

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Move sub_hfyu_median_prediction_int16() to losslessviddsp

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