Filter frame timing to try and remove scheduler jitter.

frame-timing.c

double delta_time_filtered(double dt_nanos){
	// Warm start the filter by assuming 60 Hz.
	static double x[] = {1.6e7, 1.6e7, 1.6e7}, y[] = {1.6e7, 1.6e7, 1.6e7};
	// IIR filter coefficients. 2rd order lowpass butterworth at 1/128 the sample rate.
	static const double b[] = {6.321391700454014e-5, 0.00012642783400908025, 6.321391700454014e-5};
	static const double a[] = {1.0, -1.9681971279272976, 0.9684499835953156};
	
	// Apply IIR filter coefficients.
	double value = b[0]*dt_nanos;
	for(uint i = 2; i > 0; i--){
		x[i] = x[i - 1]; y[i] = y[i - 1];
		value += b[i]*x[i] - a[i]*y[i];
	}
	x[0] = dt_nanos; y[0] = value;
	
	// Quantize the delta time, truncating towards the filtered value.
	return trunc(dt_nanos/value - 1)*value + value;
}