Return a `sc_pb.Action` with the camera movement filled.

def camera_action(self, mouse_pos):
    """Return a `sc_pb.Action` with the camera movement filled."""
    action = sc_pb.Action()
    action_spatial = mouse_pos.action_spatial(action)
    mouse_pos.obs_pos.assign_to(action_spatial.camera_move.center_minimap)
    return action