Blocking scene graph nodes outside the scene

[uwerat]

A further optimization would be to block scene graph nodes, when the item is outside the scene geometry. Already created nodes could be deleted, once an items has left the scene.

An use case, where many items might be outside is QskScrollArea.

The challenge with this feature is that the geometry of an item is relative to its parent and the criterion for blocking depends on the geometry of all ancestors. Running recursively over all children on geometry changes seems to be too expensive.

[uwerat]

With Qt 6.3 we have a new Flag QQuickItem::ItemObservesViewport, that could help to address this.

[uwerat]

Duplicate for #281