Fix 2D objects in 3D affecting bounding box and thus causing flickering of automatic pinhole plane distance

[Wumpf]

What

If you have only a pinhole camera with some 2D objects in your scene, the viewer wasn't able to decide where to place that pinhole because the only source of bounding box information were those projected 2d elements.

We now simply ignore 2d visualizers when putting together the scene bounding box - since a little while this is a separate step that combines bounding boxes per entity per visualizer, making this pretty easy!

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