Improve performance of the RenderLayerCompositor::OverlapMap

Improve performance of the RenderLayerCompositor::OverlapMap
https://bugs.webkit.org/show_bug.cgi?id=115063

Reviewed by Simon Fraser.

PerformanceTests:
Testing the performance of computing the overlap of 5000 layers.

* Layout/layers_overlap_2d.html: Added. Using non-composited layers, to check
that the performance on the non-composited path is not changing with this patch.
* Layout/layers_overlap_3d.html: Added. Records the time to do the layout of 5000
non-overlapping 3D layers.

Source/WebCore:
No new tests, no new functionality or behavior.

Do not use the OverlapMap in RenderLayerCompositor::computeCompositingRequirements if the layer already
has a 3D transform. This way we can avoid a potential expensive lookups when we know for sure the layer
is already supposed to be composited.

Also, added a bounding box of the overlap map, so that it can catch cases when the new layer is not overlapping
any of the previous layers. This is pretty common when having composited layers laid out in a vertical/horizontal list.

* rendering/RenderLayerCompositor.cpp:
(OverlapMapContainer):
(WebCore::OverlapMapContainer::add):
(WebCore::OverlapMapContainer::overlapsLayers):
(WebCore::OverlapMapContainer::unite):
(WebCore):
(WebCore::RenderLayerCompositor::OverlapMap::add):
(WebCore::RenderLayerCompositor::OverlapMap::overlapsLayers):
(WebCore::RenderLayerCompositor::OverlapMap::pushCompositingContainer):
(WebCore::RenderLayerCompositor::OverlapMap::popCompositingContainer):
(RenderLayerCompositor::OverlapMap):
(WebCore::RenderLayerCompositor::computeCompositingRequirements):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@167407 268f45cc-cd09-0410-ab3c-d52691b4dbfc