Switch to the translucent mode when rendering a stacking context created for an element with opacity less than 1

(StackingContext::render): Refine
(ViewCSSImp::beginTranslucent, ViewCSSImp::endTranslucent): New
(WindowImp::beginTranslucent, WindowImp::endTranslucent): New
(Canvas::Impl::beginTranslucent, Canvas::Impl::endTranslucent): Refine