Got authorization from all copyright holders

introduced in Thorvald's previous patch.

instead of relying on the one previously transmitted by the encoder.

find_spectral_pitch()

This involved adding kfft_single.[ch] that redefines kiss_fft a second time
with a different prefix. All this is still a bit of a mess now. The mask
had to be converted to 16-bit input, but we're still using floats to apply it.

copyright, so it's fine!)

pre-computed (instead of computing tons of pow() uselessly all the time)

Code much leaner now.