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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.

intruduces a bit of an encoder-decoder mismatch (Q8 in dB), but it'll be
reduced when the interals of quant_energy_mono() are properly converted to
fixed-point and oldEBands gets rounded instead of truncated.