swr: rename dither_pos to noise_pos

THis more closely matches what the field represents

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swr: add seperate in/out pointers to the noise shaping code

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swr: mark noise data for noise shaping as const

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swr: Use a temporary buffer for dither/Noise shaping when the input is read only

This avoids copying the input

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