Improvements to selection drawing.

        Clip out floating and positioned objects when drawing selection so
        that there is no "double painting" of selection when the selection spans both the space behind the positioned/floating
        object and the positioned/floating object itself.  Improves most Web pages.

        Do not paint left/right selection gaps if the object's edge is already outside the boundary (at a negative
        position).  Significantly improves the look of selections on digg.com, since it uses giant negative text-indent.

        Reviewed by Dan

        Covered by existing selection pixel tests.

        * rendering/RenderBlock.cpp:
        (WebCore::RenderBlock::isSelectionRoot):
        (WebCore::RenderBlock::paintSelection):
        (WebCore::clipOutPositionedObjects):
        (WebCore::RenderBlock::fillSelectionGaps):
        (WebCore::RenderBlock::fillLeftSelectionGap):
        (WebCore::RenderBlock::fillRightSelectionGap):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@35515 268f45cc-cd09-0410-ab3c-d52691b4dbfc