As pointed out in D15827 comment, the unique_ptr usage in
ShaderNodetreeWrap related code does not sound very useful. Looking at
it, whole ShaderNodetreeWrap does not make much sense - it's only
ever created, and then immediately just one thing is fetched from it.
This very much sounds like "a function", so make it just that -
header file contains just a `create_mtl_node_tree` function, and the
whole implementation is hidden from the users. Which I've also
simplified into just a handful of freestanding functions.
No functionality or performance changes, but the code does get ~80
lines shorter.
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