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

djdeath/clutter,ebassi/clutter,kerrickstaley/clutter-vala,djdeath/clutter,kerrickstaley/clutter-vala,collects/clutter,djdeath/clutter-android,djdeath/clutter-android,heysion/clutter-clone,Distrotech/clutter,djdeath/clutter,heysion/clutter-clone,kerrickstaley/clutter-vala,jigpu/clutter,djdeath/clutter-android,GNOME/clutter,djdeath/clutter,collects/clutter,kerrickstaley/clutter-vala,GNOME/clutter,Distrotech/clutter,djdeath/clutter-android,ebassi/clutter,Distrotech/clutter,djdeath/clutter,Distrotech/clutter,Distrotech/clutter,collects/clutter,collects/clutter,heysion/clutter-clone,heysion/clutter-clone,collects/clutter,Distrotech/clutter,djdeath/clutter-android,ebassi/clutter,jigpu/clutter,ebassi/clutter,GNOME/clutter,jigpu/clutter,djdeath/clutter,heysion/clutter-clone,jigpu/clutter,GNOME/clutter,jigpu/clutter,ebassi/clutter,GNOME/clutter,jigpu/clutter