Bug 107734

A stacking container is treated just like a stacking context. That is, it has z-order lists, it and its descendants are stacked as a unit, and when the RenderLayerCompositor does its overlap testing, the composited regions for all layer lists take effect only once the stacking container is done being processed. This patch also adds the function RenderLayer::isStackingContainer(). Currently, this is equivalent to RenderLayer::isStackingContext(), but in future, the definition of stacking container will be broadened to encompass more than just stacking contexts. Other than the addition of this extra function, the patch is mostly comprised of name changes. Any code that used to refer to the stacking context concept, but didn't necessarily require a stacking context in the strict, CSS-sense, was switched to refer to stacking container. No functionality was changed.