feat: add pixelate and pseudo-pixelate

[RobertMueller2]

Closes: #90

This implements a pixelate tool with two modes

• regular mode with a normal block pixelation, this is, depending on selected area and image resolution, reversible
• "independent mode" or pseudo-pixelate that takes data from outside the selected area and applies distance weighting. The original area is entirely overwritten by the interpolated data. Then, the regular pixelation creates a block-pixelate look from this.

Some things are still open:

• figure out a better icon - help needed please
• reach out to flameshot guys regarding credits
• double check if south and east buffers aren't actually off by one
• figure out if BLOCKSIZE is good at a value of 32. This may actually need to be adjustable.

Known limitations:

• in pseudo-pixelate mode, canvas background may contribute to the mix of pixels if the selection is started at image origin
• in idependent mode, if the selection "leaves" Satty's window, nothing happens. We would be lacking one fringe in that instance.

This PR was inspired by

• https://github.com/flameshot-org/flameshot/pull/3765/changes
• [needs a human pls] add pixelate functionalty (on alt) to blur #468 by kikeijuu (superseded by this PR)

Regarding the credits, the idea with taking "fringes" was taken from said PR, but I reimplemented the gist of it according to my understanding. I did, for example, skip any additional noise which might be suitable for adding later. Also, actually iterating the source (fringes) and target data (overwritten area) works a bit different due to the objects used. I don't think we need to talk about a separate licence for this bit, but we'll see what the flameshot guys say about this.

pixelation2.mp4

[RobertMueller2]

for brighter images, the "independent mode" result is a bit too bright. Maybe the weighting math isn't great yet.

EDIT: need to divide everything by two if I have a a horizontal and a vertical influence, they each add to 1. that looks a lot better. Unfortunately, now I have to redo the recording.

[RobertMueller2]

Perhaps we also really need to call this "pseudo-pixelate", no need to re-invent lingo? 😉