Auto merge of #3367 - gw3583:pc-opt, r=kvark
Auto merge of #3367 - gw3583:pc-opt, r=kvark

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.

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