fft2.FFT: restore expecting scaling behavior when calling to FFTMethods

Noticed by Stephan Preibisch:
The recent changes I introduced to the class, aimed at making it
prossible to control the number of threads used for computation,
introduced some mistakes related to the expected scaling of the results.

When using FFTMethods calls in the FFT class, the scaling must be as
following:
- calls to complexToComplex(): if forward=true then scaling=false
  if forward=false then scaling=true.
- complexToReal(): scaling is always true.
- realToComplex(): scaling is always false.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>