Compositing overlap testing can throw layers into compositing when they should not be.

Compositing overlap testing can throw layers into compositing when they should not be.
https://bugs.webkit.org/show_bug.cgi?id=50192

Reviewed by Simon Fraser.

Source/WebCore:

The previous overlap map behavior was that a non-composited query
layer would become composited due to overlap if and only if the query
layer's absolute bounds overlapped the absolute bounds of some other
layer which:
    - draws before the query layer
    - is or has a compositing ancestor

This behavior, while correct, was too permissive in throwing layers
into compositing, causing many layers to get their own backing when
they could have just gone into their compositing ancestor's backing.

The correct logic is that non-composited query layer needs to be
composited due to overlap if and only if the query layer's absolute
bounds overlap the absolute bounds of some other layer which:
    - draws before the query layer
    - has a different compositing ancestor than the query layer
    - is or has a compositing ancestor that is a descendent of the
      query layer's compositing ancestor

This patch changes the semantics of the overlap map to enable this
behavior.

Rather than having one global overlap map, there is now a stack of
overlap maps. New (empty) overlap maps are pushed onto the stack
whenever a layer becomes a compositing ancestor and popped after all
of the compositing requirements for that layer's children have been
computed.

The compositing ancestor and all of its non-composited children of a
compositing ancestor do not get considered for overlap until their
composited ancestor has been popped off the stack. If a compositing
ancestor has a compositing subtree, then any descendents of that
compositing ancestor that draw after that subtree will consider
everything in the compositing subtree for overlap.

Test: compositing/layer-creation/stacking-context-overlap.html

* platform/graphics/Region.cpp:
(WebCore::Region::intersects):
(WebCore):
* platform/graphics/Region.h:
(Region):
* rendering/RenderLayerCompositor.cpp:
(RenderLayerCompositor::OverlapMap):
(WebCore::RenderLayerCompositor::OverlapMap::OverlapMap):
(WebCore::RenderLayerCompositor::OverlapMap::add):
(WebCore::RenderLayerCompositor::OverlapMap::contains):
(WebCore::RenderLayerCompositor::OverlapMap::overlapsLayers):
(WebCore::RenderLayerCompositor::OverlapMap::isEmpty):
(WebCore::RenderLayerCompositor::OverlapMap::popCompositingContainer):
(WebCore::RenderLayerCompositor::OverlapMap::pushCompositingContainer):
(WebCore::RenderLayerCompositor::addToOverlapMapRecursive):
(WebCore::RenderLayerCompositor::computeCompositingRequirements):
* rendering/RenderLayerCompositor.h:
(RenderLayerCompositor):

LayoutTests:

* compositing/layer-creation/stacking-context-overlap-expected.txt: Added.
* compositing/layer-creation/stacking-context-overlap.html: Added.
* compositing/layer-creation/stacking-context-overlap-nested-expected.txt: Added.
* compositing/layer-creation/stacking-context-overlap-nested.html: Added.
* platform/chromium/test_expectations.txt:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@109851 268f45cc-cd09-0410-ab3c-d52691b4dbfc