Auto merge of #9756 - mrobinson:flat-display-lists-webrender, r=pcwalton

Flatten display list structure

Instead of producing a tree of stacking contexts, display list
generation now produces a flat list of display items and a tree of
stacking contexts. This will eventually allow display list construction
to produce and modify WebRender vertex buffers directly, removing the
overhead of display list conversion.  This change also moves
layerization of the display list to the paint thread, since it isn't
currently useful for WebRender.

To accomplish this, display list generation now takes three passes of
the flow tree:

        1. Calculation of absolute positions.
        2. Collection of a tree of stacking contexts.
        3. Creation of a list of display items.

After collection of display items, they are sorted based upon the index
of their parent stacking contexts and their position in CSS 2.1
Appendeix E stacking order.

This is a big change, but it actually simplifies display list generation.

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