Implement face flat/smoothing shading

Really I guess this is implementing non-smoothing, since we previously
treated all faces as if they had smoothing enabled.

It's technically a breaking change, but should make Plasma with runtime
lighting match what you'd expect from the Blender 3D viewport.
It also matches how PlasmaMax treats meshes by default with no smoothing
groups attached.

korman/exporter/mesh.py

@@ -190,6 +190,7 @@ class MeshConverter:
         for i, tessface in enumerate(mesh.tessfaces):
             data = geodata[tessface.material_index]
             face_verts = []
+            use_smooth = tessface.use_smooth
 
             # Unpack the UV coordinates from each UV Texture layer
             # NOTE: Blender has no third (W) coordinate
@@ -229,7 +230,14 @@ class MeshConverter:
                     source = mesh.vertices[vertex]
                     geoVertex = plGeometrySpan.TempVertex()
                     geoVertex.position = hsVector3(*source.co)
+
+                    # If this face has smoothing, use the vertex normal
+                    # Otherwise, use the face normal
+                    if use_smooth:
                         geoVertex.normal = hsVector3(*source.normal)
+                    else:
+                        geoVertex.normal = hsVector3(*tessface.normal)
+
                     geoVertex.color = hsColor32(*vertex_color)
                     geoVertex.uvs = [hsVector3(uv[0], 1.0 - uv[1], 0.0) for uv in uvws]
                     data.blender2gs[vertex][coluv] = len(data.vertices)