This patch allows WR to dynamically choose whether picture caching
is enabled per-frame, rather than only during initialization.
This will allow mobile devices to disable picture caching during
pinch zoom events, where tiles will be constantly invalidated.
This can also be controlled via a debug flag, which allows dynamic
toggling of picture caching in both Gecko and Wrench. This is
useful for profiling, to compare direct rasterization to picture
caching performance.
The native compositor interface relies on picture caching. So, if
a native compositor is enabled, picture caching is force enabled.
Differential Revision: https://phabricator.services.mozilla.com/D51211
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extra : moz-landing-system : lando
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