Convolution is slow

[targos]

There are probably multiple places where we can improve it.

Biggest bottleneck is the border handling (70% of the time is spent there):

https://github.com/image-js/image-js-typescript/blob/d3f0d998c623259c71d795e0abb9d6925fca89ba/src/filters/convolution.ts#L192-L268

Tested with:

import { Image } from 'image-js';

const img = new Image(2000, 1000);

img.gaussianBlur({ sigma: 10 });

console.time('blur')
img.gaussianBlur({ sigma: 10 });
console.timeEnd('blur')

console.log(img);

[targos]

First improvement: image-js/image-js-typescript#336

[Xstoudi]

Still seem slow.
At some point, could we consider using WebGPU when supported in environment to handle heavy operations?