Performance checking

[llewelld]

On machines that support shaders but with limited GPU power, it's possible the shaders will run slowly and cause the viewer's machine to run badly. I've not been able to simulate this, but it seems like a very distinct possibility.

Consequently shaderback.js should really monitor the screen update rate and deactivate itself if it drops below a certain rate over a certain period of time.

Could either be implemented as a cyclic buffer of refresh times, or triggered using a pair of low/acceptable refresh rate counters.