Adjust blur offset behind plasmoid to match plasmoid itself (second try)
Previous fix somehow did not account for (literally) edge cases, such that the background became transparent near right and bottom edges of containment.
This patch rewrites and centralizes sizing calculations, optimizes attached properties access, removes redundant IDs, replaces OpacityMask::maskSource ShaderEffectSource with a simple clipped Item, and avoid being smart about reparenting to plasmoid.
See also: 0bbba7a7
I was wrong, Mart was on the right way, this patch finally makes it as good as it was supposed to be.
Note that it still does not account for neither scale nor rotation. Those will be done later in follow-up MRs.