Currently the corner_vert and corner_edge attributes go through the
generic custom data interpolation, only to be overwritten directly
afterwards. In a profile of the execution of the subdivision surface
modifier, 5% of the time was spent in `layerInterp_propInt`. To fix
this, first build a `CustomData` layout to be used just for face corner
attribute interpolation. Then allocate the topology attributes
separately and add them to the result mesh later. This requires a fair
amount of boilerplate right now, but it should be improved with various
changes to mesh attribute storage in the future.
In a file with many object with a subsurf modifier, this change
improved the overall execution time by between 5 and 10% in my tests.
Pull Request: https://projects.blender.org/blender/blender/pulls/123254
31a67805fe