[opus] Query regarding FFT in Opus

[Timothy B. Terriberry]

Mahantesh Belakhindi wrote:
> I want to know if the KISS FFT in opus can be replaced with fast FFT.
> What are the implications & things to be taken care if this is done.

Yes. The main thing to watch out for is to make sure the scaling matches 
(and that your fast FFT supports non-power-of-2 sizes, of course).

> Also, I saw in FFT code comments that in-line FFT not supported. Please
> let me know the reason for this.

This is merely a limitation of our KISS FFT implementation, because the 
bitreverse permutation is not done in-place.