(if STATIC_MODES is defined) instead of attempting to modify read-only
storage.

slope is required).

compiled without having to use the initialisation code.

make sure never to end up with a pitch vector with >1 norm.

This reverts commit a1bc18a3.
I think it's easier for now to deal with two functions here, even though they
could be merged.

variable instead of the angle. Should now be both less CPU-intensive and
makes fixed-point code simpler.

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.