Polish initial GPGPU CUDA/GPUmat compute support.

A new PsychImaging() subfunction 'UseGPGPUCompute' must be called to
detect+select+initialize the GPGPU computing api before it can be used
by PTB. Currently only the GPUmat toolbox for NVidia CUDA computing
with Matlab is supported, but the PsychImaging et al. basic interface
should be sane enough to allow to extend this eventually to OpenCL and
octave.

Demos and help texts have been updated and cleaned up.
This is good enough for the initial release, but much work remains to
be done to make this nice and extend its useful functionality beyond a
bit of FFT based image filtering.
BitsPlusPlus: Add query for color conversion mode of a window in Color++/C48 mode.

mode = BitsPlusPlus('GetColorConversionMode', win); allows to query the
current conversion mode for window 'win' for a window in Color++/C48
mode. The function returns a -1 for "undefined' in other display modes.