Same uip in multiple subpresentation entries

Listing the same .uip file in multiple subpresentation entires should lead to running the scene only once and using the same one texture for all those subpresentations. This is probably not the case right now.

Update: This above is actually wrong.

Two subpresentation entries refering the the same .uip file must still create two independent presentations and scenes, because the sizes of the layers displaying them may differ. If interaction is involved, then that's another reason to keep them separate.

Now, there can be cases when there is only one instance of the presentation/scene wanted. However this has to be communicated by the .uia file since the loader (Q3DSWindow in our case) cannot tell the intended usage, and there is no attribute for this at the moment.