function DriftDemo5(angle, cyclespersecond, f, drawmask)
Display animated gratings using the new Screen(‘DrawTexture’) command.
The demo shows two drifting sine gratings through circular apertures. The
1st drifting grating is surrounded by an annulus (a ring) that shows a
second drifting grating with a different orientation.
The demo ends after a key press or after 20 seconds have elapsed.
The demo uses alpha-blending and color buffer masking for masking the
gratings with circular apertures.
angle = Angle of the grating with respect to the vertical direction.
cyclespersecond = Speed of grating in cycles per second. f = Frequency of
grating in cycles per pixel.
drawmask = If set to 1, then a gaussian aperture is drawn over the grating