Draw a curve with the mouse. Same as MouseTraceDemo, but asks
Screen(‘Flip’) to not clear the framebuffer after flip. This way,
we don’t need to redraw the whole mouse trace in each frame.
Uses imaging pipeline via PsychImaging for optimal performance
on modern hardware, as opposed to old style method shown in
MouseTraceDemo2, which was appropriate before the year 2007.
4/1/2013 mk Derived from MouseTraceDemo2.