https://bugs.webkit.org/show_bug.cgi?id=21049, opacity and position:relative fail in multi-column layouts.
        
Reviewed by Oliver Hunt.

Make layers split painting across columns just as normal flow content does.  This is actually very
challenging, since intermediate multicol layers can exist between a child that has to be split and
its enclosing stacking context.
        
In order to properly paginate layers, a bit is set on all pagination roots, i.e., layers that will get
split and thus split any descendant layers that they are also responsible for painting.  For a paginated
root, we walk up the layer hierarchy and collect all intermediate multicol blocks between the child
and the enclosing stacking context.
        
We then recur from the outside in, applying clipping and translation as we break up the layer into strips.
        
Composited layers remain unsplittable and have a hacked offset still in order to be placed in the right
column.
        
Painting and hit testing of columns has been simplified in all of the functions to use the x position of
the column rect instead of trying to increment an x offset by adding in widths and column gaps.  This
makes those functions directionality-independent (and the LTR/RTL code paths are now the same).
        
Fix repainting of columns as well to make sure rects are split across columns only after relative position
and transforms have been applied.
        
Added fast/multicol/layers-split-across-columns.html

* rendering/RenderBlock.cpp:
(WebCore::RenderBlock::paintColumnContents):
(WebCore::RenderBlock::adjustRectForColumns):
(WebCore::RenderBlock::adjustForColumns):
* rendering/RenderBlock.h:
* rendering/RenderBox.cpp:
(WebCore::RenderBox::computeRectForRepaint):
* rendering/RenderLayer.cpp:
(WebCore::RenderLayer::RenderLayer):
(WebCore::RenderLayer::updateLayerPositions):
(WebCore::RenderLayer::updatePagination):
(WebCore::RenderLayer::updateLayerPosition):
(WebCore::RenderLayer::paintLayer):
(WebCore::RenderLayer::paintList):
(WebCore::RenderLayer::paintPaginatedChildLayer):
(WebCore::RenderLayer::paintChildLayerIntoColumns):
(WebCore::RenderLayer::hitTestLayer):
(WebCore::RenderLayer::hitTestList):
(WebCore::RenderLayer::hitTestPaginatedChildLayer):
(WebCore::RenderLayer::hitTestChildLayerColumns):
* rendering/RenderLayer.h:
(WebCore::RenderLayer::):
(WebCore::RenderLayer::isPaginated):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@59784 268f45cc-cd09-0410-ab3c-d52691b4dbfc