This is a largely uninteresting patch that just uses the DisplayListBuilder
directly. A wonderful cleanup patch will come after this. One of the more
interesting pieces is the use of PushBuiltDisplayList. This is needed for
handling empty transactions. See https://github.com/servo/webrender/pull/934
for more info.
This basically just splits the enum in two and does the necessary plumbing. The
worst part is that now DisplayItemLayers need to have two arrays of commands.
Fortunately, this will be going away in the future.
This makes it so we don't send the child side commands to the parent.
