Previously, `writedata` was used to store `bNodeSocket->default_value`.
There are a couple of issues with that:
* Breaks if someone tries to load the file on a big endian system (afaik).
* There is a `/* do not use for structs */` comment on `writedata`.
* Depends on `MEM_allocN_len` which should be avoided in my opinion.
* Now it is more apparent, that this should be handled by callbacks as well.
The part in `readfile.c` should work just fine still. I could also do a case
distinction there, but the code would be the same for every case for now.
Just `sock->default_value = newdataadr(fd, sock->default_value);`.
This might change, if we want to store more complex socket type specific data
that does not fit into a single struct.
Differential Revision: https://developer.blender.org/D7039
Reviewers: brecht
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