kPsychExternalDisplayMethod – Create onscreen window not used for actual visual stimulation.

This flag can be passed to the optional ‘specialFlags’ parameter of
Screen(‘OpenWindow’, …) or PsychImaging(‘OpenWindow’, …).

It will cause the onscreen window to be marked as not used for actual
visual stimulation of the subject. Instead some mechanism external to
Screen() is used for displaying actual stimuli to the test subject. This
could be a display window driven by the Vulkan/WSI display backend on
suitable system setups, or some special purpose display hardware with its
own display SDK, or some VR compositor for certain VR/AR headsets.

This tells Screen() that certain aspects of this window do not matter
very much, e.g., timing precision, color/contrast precision, etc.,
because some other entity is responsible to ensure those properties for
all visual stimuli drawn into this onscreen window, e.g., an external
VR driver or our PsychVulkan driver.

In practice this means that Screen() can skip certain - otherwise
essential - calibrations and startup checks, runtime checks, and various
warning or status messages during startup or runtime of a session,
because the calibrations are not needed for performing properly and
failing those checks or calibrations won’t matter at all for the success
of the session, or the status/warning messages would be nothing more than
distracting or misleading clutter to the experimenter.

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