Grid8x8 FFT procesessing example: Add an helper
function to filter NaN values in FFT filter arrays and make use of it.

Signed-off-by: Piment Noir Task Force <elecdev@piment-noir.org>
Grid8x8 FFT processing example: reduce global
variables to the bare minimum to ease factorization.
Signed-off-by: Piment Noir Task Force <elecdev@piment-noir.org>