Fix #140225: Always ensure mesh topology is up to date during USD import

It is possible for a mesh to change topology across frames but still be
detected as not needing a topology update.

Until we can make a finer-grained check against the before and after
topology, unconditionally ensure it's updated for now.

Adds a new test that checks a few frames of changing topology that is
similar, but not the same.

Pull Request: https://projects.blender.org/blender/blender/pulls/140253

tests/python/bl_usd_import_test.py

@@ -160,6 +160,32 @@ class USDImportTest(AbstractUSDTest):
         self.assertEqual(len(mesh.vertices), 5)
         self.assertEqual(len(mesh.polygons[0].vertices), 5)
 
+    def test_import_mesh_topology_change(self):
+        """Test importing meshes with changing topology over time."""
+
+        infile = str(self.testdir / "usd_mesh_topology_change.usda")
+        res = bpy.ops.wm.usd_import(filepath=infile)
+        self.assertEqual({'FINISHED'}, res, f"Unable to import USD file {infile}")
+
+        # Check topology for all frames against expected vertex and face counts
+        expected_face_verts = [
+            (4, 4, 4),
+            (3, 4, 5),
+            (3, 3, 6),
+            (4, 4, 4),
+        ]
+        for frame in range(1, 5):
+            bpy.context.scene.frame_set(frame)
+            depsgraph = bpy.context.evaluated_depsgraph_get()
+
+            mesh = depsgraph.objects["TopoTest"].data
+
+            expected = expected_face_verts[frame - 1]
+            self.assertEqual(len(mesh.polygons), len(expected), f"Unexpected data for {frame=}")
+            for face in range(0, 3):
+                verts = mesh.polygons[face].vertices
+                self.assertEqual(len(verts), expected[face], f"Unexpected data for {frame=} {face=}")
+
     def test_import_mesh_uv_maps(self):
         """Test importing meshes with udim UVs and multiple UV sets."""