Similar to previous commits, avoid using pointers that are redundant
to their corresponding index. Also avoid storing the edge vectors stack
in the data-per-loop when it can just be a function argument.
Face corner normals are calculated when auto smooth or custom
per-corner normals are used.
This brings the per-face-corner data from 64 to 24 bytes, as measured by
`sizeof`. In a large test file I observed a 20% performance improvement,
from 285 to 239 ms.
9e9ebcdd72