Refine adaptive "speed optimization" to have faster initial render.

[MatthewPeterKelly]

Follow-up to #119, which added support for dynamically adjusting the render quality to get faster frame rates. That implementation just hard-codes the speed optimization level at 2... but we could do better.

We want to control frame rate, and we have the "speed optimization parameter" as the control knob. I think we can use a simple controller to achieve the desired results here:

if (measured_frame_rate < lower_threshold) {
    optimization_level++;  
} else if (measured_frame_rate > upper_threshold) {
    optimization_level = max(0, optimization_level-1);
}

Using a large deadband should help with chatter, but we might also want to filter the measured framerate. Probably a simple first-order IIR filter would work... but we could also use a moving average. We'll need to do some experiments!

[MatthewPeterKelly]

Done! #145

Almost. There is one thing left to do -- if the initial render is really slow, then we can be non-responsive right when the application starts, which is no good. The way to fix this, I think, is to have a "max render time", after which the render just aborts and tries again with a higher optimization value (with some safety to never give up if the render command is "as fast as possible" already).