windowManager: Complete interrupted size change effects

Resizing effects are more finicky as other effects, as the actual
animation is delayed until we receive the ::size-changed signal.

However that signal may never be emitted if the window is destroyed
just after starting the size-change effect, in which case the effect
is never completed, blocking mutter from destroying the corresponding
window actor.

Address this by tracking when a resize effect is pending, and complete
the effect when appropriate.

mutter#655
!815