Previous code would re-create a node-tree everytime conversion code was
called (on main file read, but also on all link/append operations).
Worse, it would assing a local generated nodetree to converted linked
GPv3 objects.
This commit solves both issues in a similar way as what was done for the
legacy mesh 'auto smooth' generated node tree.
It also slightly refactors the conversion code by adding a single struct
containing all 'runtime' conversion data (two mappings currently). This
struct is then passed to internal conversion functions as reference.
c3ee098c78