This requires adding another notion of mouse-release events to `ClipEvent`. We now have four:
* `MouseUp` - the mouse was released, any object on the render list can handle this event ("anycast" event)
* `MouseUpInside` - the mouse was released inside this display object, only the mouse-picked target of the event can handle it
* `Release` - the mouse was released inside the last clicked display object
* `ReleaseOutside` - the mouse was released outside the last clicked display object
For those keeping score at home, in AVM2, the valid progression of events is either...
* On the same object, `mouseDown`, `mouseUp`, and `click`
* On one object, `mouseDown`, then some mouse movement that takes the cursor out of the first object, then on another object `mouseUp`, and then finally the first object gets `releaseOutside`.
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