slow font handling in qtquick applications with threaded rendering

when using threaded rendering the `QFontPrivate::engineForScript` function will always destroy and reload the font database (`engineForScript` is accessed via `updatePaintNode`). disabling the threaded rendering this won't be done. this is a rather expensive operation (at least on linux) and afaict the ping-pong between application and render thread will alway cause the font-db to be rebuilt when `Text` items are created or the font properties are modified