The is following: split copy on write update for node trees, and if we are only
tagging for uniform buffer update we skip whole datablock copy and only invoke
copy default_values form original nodetree to a copied one.
Thing which i'm not sure is: whether we need to use different branches in graph
itself to control such a conditional behavior, or whether we need to store tag
somewhere in the dependency graph. There are obviously cons and pros in both
approaches, and need to think about this. Maybe with more examples it becomes
more obvious which way is better.
This only fixes manual tweaks for now, animation support is coming.
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