Once this patch lands, all content drawn by WebRender is drawn into
a picture cache surface.
This will incur some extra GPU memory overhead since there are extra
GPU texture buffers. Much of this can be reduced by adding a couple
of simple optimizations in future to detect tiles that are solid
colors only.
With this change, we'll now be able to provide exact dirty rects for
the entire screen without any hacks, and start the work to draw into
OS compositor surfaces directly.
Differential Revision: https://phabricator.services.mozilla.com/D47395
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extra : moz-landing-system : lando
e861c795b9