This prevents access to non-existent typeinfo during type initialization,
when node types have been removed and such nodes are deleted from older files.
All blenkernel functions now only set the node->update flag instead of directly
calling the update function. All operators, etc. calling blenkernel functions
to modify nodes should make a ntreeUpdate call afterward (they already did that
anyway).
Editor/RNA/renderer/etc. high-level functions still can do immediate updates by
using nodeUpdate and nodeUpdateID (replacing NodeTagChanged/NodeTagIDChanged
respectively). These old functions were previously used only for setting
compositor node needexec flags and clearing cached data, but have become generic
update functions that require type-specific functionality (i.e. a valid typeinfo
struct).
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