RenderSettings.OnDemand not working in Qt6

RenderSettings.OnDemand is not working in Qt6 and
there's no equivalent functionality to control FPS at the moment.

Below is a note from discussion.
Qt3D sets a dirty flag when there's a change in 3D scene in order to notify QtQuick to render.
However since Qt 6.0, the QtQuick render backend doesn't honor the dirty flag anymore and renders the frame regardless of the dirty flag being set.
This means there's no way of throttling QtQuick to only render when something in Qt3D has changed in Qt 6.0. Note that the Qt3D content is not redrawn every frame (with OnDemand) but QtQuick redraws all the QQuickItems + the texture rectangle that contains the last rendered 3D content all the time.