* Fix polygon not converting correctly to cloze
* Fix first polygon disappearing when creating se...
...cond one during editing
Previously, a fabric object was passed directly to a `Shape` such as
`Rectangle` or `Polygon`, so mutating a non-primitive property of the
shape would lead to mutating the original fabric object as well.
* Commit addition of polygon immediately
Unlike the rect or ellipse tools, when the polygon tool was active,
clicking on the canvas did not fire the `object:removed` event and the
`change` event was not dispatched. As a result, an addition of a polygon
was not saved to the DB when switching to another note or closing the
editor in edit mode without performing an action that dispatched the
`change` event.
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