feat(effect): support kdecoration2 blurregion

Use kdecoration2 blurred regions to adjust blur effect appropriately when
needed.

This is based on [1] but in contrast comes without the windowDecorationChanged
signal in the effects handler which looks to be redundant as the decoration
change does not affect the original blur region.

[1] https://invent.kde.org/plasma/kwin/-/commit/bc145b61



Co-authored-by: Michail Vourlakos <mvourlakos@gmail.com>