Add support for radius input parameter to Blur function

[bgreenblatt]

I have a requirement to supply the radius instead of calculating it based on the sigma input. I implemented it like this:

-func Blur(img image.Image, sigma float64) *image.NRGBA {
+func Blur(img image.Image, sigma float64, radius int) *image.NRGBA {
        if sigma <= 0 {
                return Clone(img)
        }

-       radius := int(math.Ceil(sigma * 3.0))
+       if radius == 0 {
+               radius = int(math.Ceil(sigma * 3.0))
+       }

Let me know what you think.

[disintegration]

Indeed, it may be useful sometimes to control the radius parameter separately from the sigma, but this requires changing the signature of the function. This would break other people's code.

[bgreenblatt]

Makes sense. Maybe better to add a new API that supports both parameters and has some code duplication