Auto merge of #14084 - mrobinson:scroll_root_2, r=glennw

Don't promote all scrollable regions to stacking contexts

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Don't promote all scrollable regions to stacking contexts

Instead annotate all flows with their owning ScrollRoots. When
processing the display list items into a flattened display list, we add
PushScrollRoot and PopScrollRoot to signal when scrolling regions start
and end. It is possible for content from different scrolling regions to
intersect and when they do, the stack of scrolling regions is
duplicated.  When these duplicated scrolling regions stacks reach
WebRender, it will scroll them in tandem.

The PushScrollRoot and PopScrollRoot items are currently represented as
StackingContexts in WebRender, but eventually these will be replaced
with special WebRender display items.

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- [x] These changes fix #13529 and #13298. (github issue number if applicable).

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