Add a RendererOption to configure if picture caching is enabled.
Add a RendererOption to configure if picture caching is enabled.

Although not hooked up internally yet, this provides a hook so
that the gecko pref can be added.

We'll use this to provide a temporary way to compare performance
and also to allow picture caching to be disabled temporarily if
we encounter any bad regressions.

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender