Fixes#115776
Create a minimal structure that contains the instructions necessary to
copy from one custom data format to another. This structure is similar
to the one used in dfacaf4f40. It should have been used in
9175d9b7c2, which instead introduced quadratic performance
relative to the number of layers when copying every element.
In this commit, copying the entire mesh and adding new custom data
are explicitly changed to use the new map to speed up copying many
elements at a time.
The non-map attribute copy functions are also changed to check for when
the source and result BMeshes are the same. In that case it's much
faster to call the "same format" function from 9175d9b7c2.
For numbers, the timings are arbitrarily influenced by how many layers
I add in my testing. With 50 or so layers, a 10x difference is easily
observable though.
Pull Request: https://projects.blender.org/blender/blender/pulls/115824
0b1049b41d