Constrain height of BackgroundParallaxHeaderModifier

[LePips]

Take a look at BackgroundParallaxHeaderModifier and find a way to constrain the height of the header along with the scroll view offset. For iOS item views the headers have a bottom color element which goes the length of the view and shows when the overlaying content is not long enough and scrolled to the bottom.

iOS - example

Criteria: the header frame is constrained based on the given height and the scroll view offset. May require some refactoring in the scroll views such that the header height is properly given.