Preference is named "WebKitAcceleratedCompositingEnabled"
        and is a boolean value. When false, prevents compositing layers from
        being created, which prevents hardware animation from running.
        Also forces video to do software rendering. Added a cache for
        the flag in RenderLayerCompositing and made it all work
        on-the-fly when the flag is changed while a page is loaded.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@45199 268f45cc-cd09-0410-ab3c-d52691b4dbfc