[BUG] not support the image with alpha channel

Nobody has claimed this yet.

this bug is in the original ver. ( http://www.quasimondo.com/StackBlurForCanvas )

If the image has alpha channel , the edge of image wlil be light.

It's try to solve this issue , but do sth. is wrong.

I fix it in my simple blur project: https://github.com/finscn/SimpleBlur

the key code is (in doStack method)

```
var _a = ((a_sum * mul_sum) >> shg_sum);
if (_a === 0) {
_r = _g = _b = 0;
} else {
k = 255 / _a;
var _r = ((r_sum * mul_sum) >> shg_sum) * k;
var _g = ((g_sum * mul_sum) >> shg_sum) * k;
var _b = ((b_sum * mul_sum) >> shg_sum) * k;
}

... ...
var npa = pixels[np + 3];
k = npa / 255;
var npr = pixels[np] * k;
var npg = pixels[np + 1] * k;
var npb = pixels[np + 2] * k;
```

Start by locating the doStack method in the JavaScript port and reproduce the light edge with an image that has an alpha channel. Compare its premultiplied-alpha handling with the referenced SimpleBlur implementation and the original StackBlurForCanvas code. Done means transparent or semi-transparent image edges no longer become light after blurring, with the behavior checked against an alpha-channel input.

Written by the indexing model from the issue text.