examples: move OBJ-related files to subdirectory

pathtracer/examples/cornell-box.yaml

@@ -17,6 +17,8 @@ lights:
       color: {r: 1.0, g: 1.0, b: 1.0}
 
 meshes:
-  - obj_file: "pathtracer/examples/cornell-box.obj"
+  # FIXME: make the path relative to the YAML in some way?
+  # Easiest solution would be to chdir to the YAML's directory
+  - obj_file: "pathtracer/examples/objs/cornell-box.obj"
     translation: [0.0, 0.0, 1.0]
     rotation: [0, 180, 0]