keyframe-transition: Create a new interval if necessary

keyframe-transition: Create a new interval if necessary

When setting up the transition manually by calling

  clutter_keyframe_transition_set_key_frame (transition, n, keys);
  clutter_keyframe_transition_set_values (transition, n, values);
  clutter_keyframe_transition_set_modes (transition, n, modes);

the frame doesn't have a valid interval when calling set_keys(), so we
need to check its existence and create it if necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=676031