PushStackingContext has its own bounds field, instead of using common DisplayItem rect

[vvuk]

push_stacking_context sets DisplayItem::rect to Rect::zero() always, and has its own bounds inside StackingContext. I think this is a leftover from when SC's were converted to push/pop; we should be able to get rid of the inner bounds, right?

[mrobinson]

@vvuk This seems reasonable to me.

[glennw]

@mrobinson Is this still relevant?

[mrobinson]

@glennw This shouldn't be an issue any longer because now stacking contexts just have a bounds and no longer a clip bounds as well. That bounds is stored in the common DisplayItem struct, so I'll just close this.