rendering a lot if prefixes are switched very often. this will make
possible for instance to render the background of taskbar items with a
PanelSvg.
This behaviour is disabled by default, you can enable/disable it with
setCacheAllRenderedPanels(bool), get with cacheAllRenderedPanels() and
erase the entire cache with clearCache()
svn path=/trunk/KDE/kdebase/workspace/libs/plasma/; revision=799411