PsychImaging: Handle dual-display side-by-side on AMD 16 bpc framebuffer properly.

In a classic "stereo presentation" dual-display side-by-side configuration,
odd scanlines of the x-framebuffer encode the content for the left video
output (viewport offset (x,y)=(0,0)), whereas the even scanlines of the
x-framebuffer encode content for the right video output. Ie., what was dead
stride on single output now becomes storage for the 2nd output.

Therefore we encode the proper horizontal sampling offset for the onscreen
windows input image texture for the shader to sample when computing output
for even rows - Done in PsychImaging.m. The shader samples and processes
accordingly instead of just zero-filling and skipping "dead even rows".

Performance note:

We continue to use one shader for dual- vs. single-display, as i don't
expect large performance gains for separate shaders. The single display
shader would have had to evaluate both code pathes for even/odd lines
anyway, as pixels from both even and odd output scanlines are located in
the same 2x2 pixel quad, so likely the hw would have had to execute both
diverging branches and just select proper end results.

-> Not yet tested on real hw, only single display tested so far.