[unity] SkeletonGraphic allow custom scale origin for Edit Layout Bounds

[HaraldCsaszar]

Reported on the forum:
https://esotericsoftware.com/forum/d/25709-pls-help-me-how-to-stretch-this-out/2

The RectTransform origin is used for scaling when fitting the layout bounds into the parent bounds. Unfortunately moving the origin messes with the main RectTransform, thus a separate editable Scale Origin shall be stored which is edited along with layout bounds when when in Edit Layout Bounds mode.