Sketches that use OpenGL and do not clear the background on each frame can
cause major flickering or problems when the screen clears anyway. There are
several possible solutions:

1. You may need to disable the default 2x smoothing by using
hint(DISABLE_OPENGL_2X_SMOOTH).

2. Update the drivers for your graphics card.

3. Get a decent graphics card -- the OpenGL renderer is for advanced use,
we don't support using it with cheaper built-in graphics hardware like the
Intel GMA 950.

4. If you're running Windows Vista, try disabling the Aero theme.