We can expect marginally improved loading times with this PR. Drawable, Transform and Node counts in stats panels are expected to remain unchanged - this PR does not add new scene graph optimisations, it just increases the speed with which we apply existing ones.
1. We add explicit `NodeVisitor::apply` overrides for commonly encountered node types to avoid additional virtual function calls per node associated with the default `apply` implementation.
2. We skip pushing `StateSet`s when `_mergeAlphaBlending` is enabled or the `StateSet` contains no relevant state.
3. We add a specialised variant of `CollectLowestTransformsVisitor::addTransform` accepting `MatrixTransform` to avoid matrix copies and multiplications.
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