looptris were referred to as both tris & faces, sometimes polygons
were referred to as faces too. Was especially error prone with
callbacks that took both a tri and a tri_i arguments.
Sometimes tri_i represented a looptri index, other times the corner of
the triangle from 0-2. Causing expressions such as:
`args->mlooptri[tri].tri[tri_i]`
- Rename tri & tri_index -> looptri_i.
- Rename faces -> looptris.
- Rename face_index/poly_index/poly -> poly_i.
- Declare looptri_i at the start of the loop and reuse it,
in some cases it was declared with args->prim_indices[i] being
used as well.
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