Make the naming consistent with the recent change from "loop" to
"corner". Avoid the need for a special type for these triangles by
conveying the semantics in the naming instead.

- `looptris` -> `corner_tris`
- `lt` -> `tri` (or `corner_tri` when there is less context)
- `looptri_index` -> `tri_index` (or `corner_tri_index`)
- `lt->tri[0]` -> `tri[0]`
- `Span<MLoopTri>` -> `Span<int3>`
- `looptri_faces` -> `tri_faces` (or `corner_tri_faces`)

If we followed the naming pattern of "corner_verts" and "edge_verts"
exactly, we'd probably use "tri_corners" instead. But that sounds much
worse and less intuitive to me.

I've found that by using standard vector types for this sort of data,
the commonalities with other areas become much clearer, and code ends
up being naturally more data oriented. Besides that, the consistency
is nice, and we get to mostly remove use of `DNA_meshdata_types.h`.

Pull Request: https://projects.blender.org/blender/blender/pulls/116238