Disable transform and mask display when there's no active clip.
It's not a matter of returning fallback dimensions if there's no
slip, it's also matter of making it so stabilization and distortion
routines are aware of clip == NULL which is really crappy.
Also almost all the operators are disabled in clip editor without
active clip already anyway.
Also tweaked header UI a bit to not display mask stuff when there's
no active clip,
1776cb9244