Fixed overflow

If you add more than 2^31 values to the windowed mean you get an overflow and the mean will be 0 for the next 2^31 values you add.
Actually this cannot be considered an issue for the windowed deltaTime between frames because you have to spend more than 1 year to get the overflow (assuming 60 FPS).
However, WindowedMean is a generic math util that you can use for your own stuff making it possible to trigger the overflow.
Test program:
````java
public class WindowedMeanTest {
	public static void main(String args[]) {
		WindowedMean wm = new WindowedMean(5);
		long t = (1L << 31) - 10;
		long n = t + 20;
		for(long i = 0; i < n; i++) {
			wm.addValue(i%10);
			if (i > t || i < 10)
				System.out.println(i+": "+wm.getMean());
		}
	}
}
````