Moves the bitrev step to the IMDCT pre-rotation
Applying the forward FFT gain up-front for fixed-point too

This makes us lose a bit of precision in the first steps, but our gain is more
precise because it's only rounded once. Overall, SNR is slightly improved.
Moving bitrev step to forward MDCT too
Getting rid of the inverse FFT entirely

IMDCT now uses the forward FFT.
Merges the FFT scaling with the MDCT pre-rotate
Reverse the ordering of the FFT stage to optimize a degenerate radix-4 case.

This also happens to increase the accuracy since it appears that the new
ordering is optimal (at least for 20 ms frames), whereas the previous ordering
was pessimal.