Draws some line into an offscreen window, displays it,
then does the same drawing into a texture.
Result should be the same, unless something is screwed wrt. texture
format, in which case one of both drawings will be flipped upside-down or
transposed etc.
This should pass on all hardware. Contributed by some unknown user
"empedia".
Psychtoolbox/PsychTests/DrawingToTextureVsOffscreenWindowTest.m