Steps through all 256 grayscale levels and uses
Datapixx et al. scanline readback to check what
the GPU actually outputs. Plots it and prints
the first 10 pixels of the topmost output scanline.
This is meant to facilitate low-level diagnosis
of GPU dithering bugs if our regular test script
BitsPlusIdentityClutTest.m reports dithering trouble
and can’t fix the problem automatically.
‘fullscreen’ Defaults to 1 for fullscreen windows.
0 = Use a windowed window at top of screen for
display to also diagnose possible compositor