What kind of windowPtr is this? Returns 0 if it's invalid, -1 an offscreen
window or a normal texture, 1 our onscreen, 2 Matlab's onscreen, 3 a
non-redrawable texture, 4 a proxy window. "windowPtr" can be an array of window
pointers, eg
Screen(
Screen('Windows'),'
WindowKind'). To count
Screen's onscreen
windows, do this: sum(1==
Screen(
Screen('Windows'),'
WindowKind')).
Please note that the current PTB doesn't have textures of type '3', i.e. all
textures are also useable as offscreen windows and vice versa. Also, it is not
possible to refer to the Matlab window itself in Psychtoolbox-3 so won't every
encounter the value '2' - it only exists for backwards compatibility with old
Psychtoolboxes.