Adapt example "examples/high-frequency.py" from glumpy to vispy

First try to adapt example "examples/high-frequency.py" from glumpy to vispy ("examples/demo/gloo/high_frequency.py").

Is it the right approach ? Mainly for transparency, timer and naming convention.

See issue #439, #435.
 
Can you add `# vispy: gallery 20` so that a screenshot of this example will be included in the Gallery. The `20` is the frame number on which the screenshot is grabbed,