Differentiate clearly how child layers handle scroll events

This allows the scroll handler to know if a child layer didn't handle
an event or the scroll position of the child layer was simply unchanged.
Don't let base layers override root layer size

Base layers (the background layer of each frame) shouldn't override the
size of their root layers. This allows base layers to scroll inside
root layer frames. This does mean that when determining the maximum
scroll position, we need to look at the size of scrolling root children
though.
Compound scrolling offsets when setting content offset

When traversing the layer tree to assign content offset, the new offset
needs to take into account any additional offset from children that are
also scrolling roots. This means that when you scroll a parent frame, it
doesn't override the scroll position of its children, but adds to it.