UI: Fix rotation handle find angle

The angle at which the rotation handle is drawn is independant from the
sceneitem's flip. As such, this changes `FindHandleAtPos` to use the rotation
angle of the item, free of its flip, instead of doing post-rotation
corrections. This fixes a bug where when the item was horizontally flipped,
the handle was not "found" at its drawn location.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f