Enable multiple View3D items with RHI-based rendering

Run the view3d example. This does not work out of the box with QSG_RHI=1.

(when having only Offscreen View3Ds, there is no problem - however when at least one Underlay or Overlay View3D is introduced, it breaks down)

There are two independent problems here: one is on the qquick3dscenerenderer.cpp level where the split prepare-render approach becomes problematic when multiple, non-Offscreen View3Ds are involved. The other is in the renderer where we try to use the same uniform buffer for all the View3Ds that reference the same shared scene.