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.
 
For consistency, can you please rename to `a_position`, `u_resolution` and `u_global_time`?