Get local rect of window or screen. This has its top-left corner always at (0,0)
and encodes the useable size of the window or screen. E.g., in certain stereo
display modes or other special output modes, the actual useable window area for
stimulus drawing may be much smaller than the real area occupied by the window.
Example: In interleaved stereo modes, the effective useable height of a window
is only half the real height of the window. Use this function to get the actual
useable drawing area for a window or screen.