Frosted Glass Effect - background blur in a scene

It is similar to QTBUG-85310 but it is more general.

"background blur" is not only for window background.

It should be supported in QQuick.Item.

It is available somehow by using Blur effect with specifying an item as a source behind.

But It requires clearly structured hierarchy of QML to split background and foreground.

Recently, glass morphism and Fluent design get popular and these need "background blur" in 2D.

These design styles should be supported in QDS like Figma

https://www.youtube.com/watch?v=-WyW2BHqVb4

https://www.youtube.com/watch?v=uRVnX0k593E

It seems QQ3D has already supported this by allowing the each scene graph node in 3D to access the framebuffer.   If we make this in 2D scene, background blur could be our strongest feature to designer.