GPUFFTVideoCaptureDemo: Add color filtering support.

Setting 'depth' == 3 in 'usegpu' mode allows filtering of
RGB color images.

Test on Ubuntu Linux 12.04.1 LTS with 8-core 2.8 GHz MacPro
and NVidia GeForce-8800 Ultra under R2007a shows:

Mono mode: 640x480@60Hz video. CPU = 8 fps, GPU > 60 fps.
Color RGB: 640x480@60Hz video. CPU = n/a  , GPU ~ 42 fps.

Pure fft compute shows that GPU method is 21x faster than CPU.

-> Not bad at all.