Simplify/Refine StereoCrosstalkReduction method a bit more.

PsychImaging() gets an additional 'method' parameter, so user can
select between different crosstalk reduction methods, if such should
become available in the future. Currently only one method is implemented,
the one of Diederick.

We drop the 'backgroundColor' parameter, as PsychImaging can directly
use the background clear color as set in the PsychImaging('Openwindow', ...);
call - after some validity checking.

-> PsychImaging and its help text updated accordingly.
-> ImagingStereoDemo updated, also a bit modernized ot use unified 0-1
   normalized color range and drop some redundant code.
-> Shader cleaned up a little bit.
-> Tested, works.