added gpu_fft code from rpi samples. not used yet.
First integration of GPU FFT. Right now, it's not really faster than
fftw3 which is most likely due to my crude, not optimized implementation
of 2d fft based on the 1d fft provided.

The thing is, I only need real->complex FFT, provided is the general
complex->complex FFT. Transposing the matrix in between takes time and
can most likely be optimized. Where is my "Algorithms in C" book when
I need it?

Anyway, I'm okay with the status as of now.