A widget that overlap two widgets and gives a parallax effect to the background one.
The effect is obtained by using a Stack and a nested OverflowBox twice the size of the main child
For this widget working it needs to find a ParallaxArea somewhere above the tree
The center of the parallax effect will be on the center of the first ParallaxArea found
If no ParallaxArea is found this widget will simply render the child and the background without any effect.