Remove the one-definition-rule violation due to two `Triangle` types
being used inside the same `blender::io::stl` namespace. Make a small
set of non-functional changes in the surrounding code along the way to
use `const` in more places.
STL had 4(!!) triangle types:
- A `class Triangle` : Used for import, contains indices
- A `struct Triangle` : Used for export, contains verts/normals, the ODR
violation
- A `struct ExportBinaryTriangle` : Used for export but is really the
same as the above type
- A `struct STLBinaryTriangle` : Used for import but was a duplicate
of the above type
Now STL has just 2. The `class Triangle` containing indices during
import and a `struct PackedTriangle` containing the verts/normals for
everything else. The duplicated `BINARY_HEADER_SIZE` constant is now
common as well.
Performance remains the same for both import/export and ascii/binary.
Pull Request: https://projects.blender.org/blender/blender/pulls/121179
4987ef010c