Optimized box blur

[RunDevelopment]

I noticed that our box blur was slower than gauss for large radii, so I know that something was wrong. We were using a cv2.filter2D based box blur. While this is correct, it's not ideal for performance. Box blur is a highly optimizable filter, so cv2.filter2D is far too general.

I used cv2.blur (which implements a highly optimized box blur) whenever possible. Since cv2.blur only supports integer radii, I only use it when the radii are integers and when the radii are so large that the difference between N.5 and N is not noticeable.

With my 2k example image, the box blur node takes this long:

Radius	Old (s)	New (s)	Change
3	0.051	0.051	1x
6	0.058	0.058	1x
10	0.067	0.06	1.1x
20	0.062	0.062	1x
50	0.11	0.062	1.8x
100	0.61	0.065	9.4x
500	0.91	0.083	11x
1000	1.2	0.1	12x