[security] replace pixelation by pseudo-pixelation

[jrpie]

Fixes #3289
Fixes #3763

Conflicts with PR #3368

Changes

Replaces pixelation by pseudo-pixelation to make recovering information with tools like unredactor impossible.
Only the fringe of the selected area is used to calculate a pixelation-like effect; the interior does not effect the output and hence can not be recovered.

Concerns

• The fringe of the selection can still be recovered (i.e. drawing a black box is still more secure)
• The look of the pixelation effect changes (as the interior of the image is no longer used as input)
• It might be better to use HSV-interpolation instead of RGB
• One might want to add an option to switch between pseudo-pixelation and (insecure) true pixelation.

Examples of the new effect