Add Ability to add backgrounds to the WaterfallGrid

[danielgruber]

This allows for example:
Having a full background color / pattern background

[paololeonardi]

Hey @danielgruber, I appreciate the time you put into creating the PR.
Can you give me a little bit of context, why do you think this is needed?

The background(_:alignment:) SwiftUI method already offers the ability to customize the background of a view with another view of any type. (https://developer.apple.com/documentation/swiftui/view/3278516-background)

i.e.

WaterfallGrid(rectangles) { rectangle in
  RectangleView(rectangle: rectangle)
}
.background(Color(red: 0.2, green: 0.2, blue: 0.2))

[danielgruber]

Hi @paololeonardi It's right this works if you want to have a Color as it won't have an effect on the scroll area. If you you want to have a pattern as a View it won't work as the background won't scroll with the view, e.g. Image(uiImage: patternImage).resizable(resizingMode: .tile)

Do you know about another option to have an "scrollable" background image? Maybe we will also have a solution from apple at WWDC 2020