This makes it explicit whether or not you are creating a ScrollFrame
when using the API. Before this was based on the content size of the
clip, which was awkward and could lead to weird situations where you
created a ScrollFrame when you didn't mean to or the reverse.

Fixes #1335.

Clip coordinates previously used a weird coordinate space relative to
the origin of the display item. This was different than every other
coordinate specified in the display list, so we smooth over this
wrinkle. To do this we need to copy out the complex clips from the
auxiliary list and adjust them so that they are relative to the scroll
clip node.

Fixes #1090.
Fixes #1408.

This is a slight revert of the removal of per-item complex clips, but
these clips are important for Servo.

This is necessary because of the changes to the API.