swr: add noise_scale field to seperate it from the noise shaping scales

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swr: reorder the operations in the noise shaping code

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swr: scale data down in noise shaping to avoid cliping

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