AdditiveBlendingForLinearSuperpositionTutorial: Fix cursor handling.

Use Show/HideCursor(screenNumber) instead of Show/HideCursor,
so it can address multiple screen's on Linux individually
to take advantage of its higher flexibility.
Screen for 32-Bit Octave on Linux rebuilt for testing.
PsychImaging: Simplify setup of Native10Bit mode.

Screen('GetWindowInfo')'s winfo.SpecialFlags setting
1024 clearly signals if our 10 bpc framebuffer hack
is in use (1024 present) or if the OS did the 10 bpc
setup (absent). Check for this value to find out if
we need to load and attach our custom GLSL ARGB2101010
formatting shader, or if we can skip this step.

-> Get rid of the tons of driver checks.