Blurring subimage

Ever considered making your effects work on a portion of an image?

Your glfx library is pretty awesome so I won't link to my semi-working version. In mine, I implemented functionality to section off a portion of an image to blur. Selecting a box or circle of an image to blur.

i.e. Image that is 400 x 600

```
blur( image, destination, blurRadius, boxTopLeft, boxTopRight )
```

or

```
blur( image, destination, blurRadius, centerX, centerY, boxRadius )
```

Start by locating the existing blur effect and its public API in the glfx.js codebase. Compare the requested rectangular and circular selection forms, then determine how partial-image coordinates should be represented. Done means blur can be limited to the requested region without changing behavior outside it, with coverage for both selection shapes.

Written by the indexing model from the issue text.