[FEAT] Add Gaussian Blur to RawImage

[BritishWerewolf]

The reason for this, is for models like U2Net where the input image size is 320x320, meaning that the resulting output needs to be resized, and can have sharp edges; blurring before resizing can smooth this out.

This is the code that I used to generate the images.

const kernels = [5, 9, 17];
const sigmas = [2, 3, 5];
const chunk = 4;
for (let i = 0; i < kernels.length; i++) {
    const kernel = kernels[i];
    const sigma = sigmas[i];
    image
        .gaussianBlur(kernel, sigma, chunk)
        then(image => image.save(`blurred-(${kernel}-${sigma}).png`));
}

Sigma should roughly be a third of kernel size (from what I found researching).
Kernel must be odd, since we want an even square around the pixel.
For example, if we had a 3x3 kernel grid, then when we place this on our pixel there is an even space all around that pixel (in this case 1 pixel).

Here is a demo of what this implementation currently looks like.

Image	Kernel	Sigma	1 chunk	4 chunks	12 chunks
	N/A	N/A	N/A	N/A	N/A
	5	2	9 seconds	3 seconds	3 seconds
	9	3	31 seconds	4 seconds	5 seconds
	17	5	1 minute, 42 seconds	8 seconds	7 seconds

EDIT: This has since been updated so that a number of chunks are not passed, and instead we will use the navigator.hardwareConcurrency property to find the ideal number of chunks.
As you can see above, there was no time save between 4 and 12 because my PC had 4 cores.

[BritishWerewolf]

The reason for this, is because a kernel is essentially a square that is applied to every pixel in an image.

Consider a grid of 5x5, it would look like this, with our pixel (P) in the middle.

. . . . .
. . . . .
. . P . .
. . . . .
. . . . .

Here, it is clear to see that there is an even spacing around the pixel.

Now, consider what would happen if we used an even kernel size of 4.

. . . .
. . . .
. . P .
. . . .

The pixel does not sit neatly in the middle of the kernel and this is a problem.

[BritishWerewolf]

This was originally a hard coded value, passed into the function, however this approach makes more sense.
Looking at the table in the opening post, it shows that (at least for my machine) there is no benefit to increase the chunks above 4.