Previously, picture caching code would use the viewport of the
scroll root to find a clipping rect for picture cache tiles. This
viewport rect was also used to eliminate fixed position clip rects
on primitives that would otherwise cause unwanted invalidations
due to them moving relative to the scroll root when scrolls occur.

Now, the picture caching code uses a similar technique to Gecko
to find shared clips on primitives in a picture cache. These clips
are filtered out from being applied on a per-primitive basis, and
instead applied once during compositing the tiles into the parent
picture.

This is a potential performance improvement, since less per-item
work is required when building clip chains. More importantly, it
means the picture caching code correctly handles cases where the
scroll root contains fixed position elements (or other scroll
roots). This is a requirement before we can enable picture caching
on multiple slices.

https://hg.mozilla.org/mozilla-central/rev/ec5ae55ac0cb

== Change summary for alert #23022 (as of Tue, 10 Sep 2019 12:33:14 GMT) ==

Improvements:

3% displaylist_mutate linux64-shippable-qr opt e10s stylo 3,447.59 -> 3,329.52
3% displaylist_mutate linux64-shippable-qr opt e10s stylo 3,438.50 -> 3,333.58

For up to date results, see: https://treeherder.mozilla.org/perf.html#/alerts?id=23022