Changing sourceItem size dynamically does not function as expected

As described in https://codereview.qt-project.org/c/qt/qtquick3d/+/324316

A somewhat special case, but is actively excercised by at least one external demo (DynamicMetersDemo)

The dev branch needs a similar solution, with some tests for this case. Right now nothing exercises changing the sourceItem's width or height dynamically, apart from that one external demo in quick3d-demos but that's not compatible with Qt 6 - ideally introduce some autotest.

It could also be interesting to check what happens in the other two cases: when the sourceItem is an Image and when the sourceItem is an Item tree with layers.enabled: true. Does it render as expected then, after changing the item's size.