Revise CSSStyleDeclarationImp::computeStackingContext to check 'opacity'.

Note a new stacking context is created for each element with opacity less than 1 by this patch.

(CSSStyleDeclarationImp::computeStackingContext, CSSStyleDeclarationImp::compute): Support 'opacity'
(CSSStyleDeclarationImp::getUsedPosition): New
(CSSStyleDeclarationImp::isAbsolutelyPositioned, CSSStyleDeclarationImp::isRelativelyPositioned, CSSStyleDeclarationImp::isPositioned): Refine
(CSSStyleDeclarationImp::resolveRelativeOffset, CSSStyleDeclarationImp::isFlowRoot): Refine
(Box::setStyle, Block::setContainingBlock): Refine
(CSSDisplayValueImp::compute): Refine
(CSSPositionValueImp::isPositioned, CSSPositionValueImp::isRelative): Remove