Auto merge of #7487 - mrobinson:paint-layer-upgrade, r=pcwalton

Have PaintLayers own StackingContexts instead of the opposite

Previously, StackingContexts might have a PaintLayer. We switch the
ownership, for several reasons:

   * We want PaintLayers to potentially contain something other
     than a StackingContext soon.
   * We want to delay the creation of PaintLayers until the last
     minute, so that we can synthesize new layers for sandwiched
     content.

This commit also implements the second goal. Instead of creating
PaintLayers during layout itself, wait until we are sorting and
layerizing a completed DisplayList.

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/7487)
<!-- Reviewable:end -->

0 files changed, 0 insertions, 0 deletions