Slightly improving the accuracy of the fixed-point MDCT downscale

Also simplifying the code
Getting rid of some negations

Since we're doing two rotations, we can invert the sign on both.
Also adding a few comments for optimizing the FFT.
Fixes C89 issue
Unifying scaling of fixed-point and float FFT
Remove a SAVE_STACK that was pasted accidentally in the previous commit
Fixed-point: slight accuracy improvement in the comb filter
Minor cleanup -- nothing to see here