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.
 
This size is too big for most screens, and the aspect ratio will be bad for the gallery. Maybe 800x600?