First, calculate the pixel position of a list
item within its ancestor Scrollable.

Use the pixel position of the list item to calculate its
percentage from the top of the Scrollable.
A list item at the top of the scrollable area should 
produce 0%, and a list item at the bottom of the
scrollable area should produce 100%.

Use the scroll percentage to calculate an Alignment.
At 0%, you want Alignment(0.0, -1.0),
and at 100%, you want Alignment(0.0, 1.0).
These coordinates correspond to top and bottom
alignment, respectively.

Use verticalAlignment, along with the size of the
list item and the size of the background image,
to produce a Rect that determines where the 
background image should be positioned.

Using childRect, paint the background image with
the desired translation transformation.
It’s this transformation over time that gives you the 
parallax effect.