As Usual...
Note that we have to edit each C-defined of those callbacks that set
Node->id pointer, instead of just moving the generic call to
`id_us_plus()` in `node_init()` below the call to
`ntype->initfunc_api()`, since that one manipulates RNA pointers, which
are supposed to already handle usercounts themselves on assignment.
The C callbacks are moving back to manipulating directly DNA, so it's
their responsability to handle user count then (Py code should do that
automatically through RNA assignement).
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