Reviewed by Eric Seidel.

        Don't do GraphicsContext save/restore just to preserve the CompositeOperator
        https://bugs.webkit.org/show_bug.cgi?id=50070

        Add GraphicsContext::compositeOperation() so we don't have to do a full
        save/restore if the only context-tainting call is setCompositeOperation().

        GraphicsContext::setCompositeOperation() now stores the op in its state
        and calls a port-specific setPlatformCompositeOperation().

        No new tests, this is an optimization.

        * platform/graphics/GraphicsContext.cpp:
        (WebCore::GraphicsContext::setCompositeOperation):
        (WebCore::GraphicsContext::compositeOperation):
        * platform/graphics/GraphicsContext.h:
        * platform/graphics/GraphicsContextPrivate.h:
        (WebCore::GraphicsContextState::GraphicsContextState):
        * platform/graphics/Image.cpp:
        (WebCore::Image::fillWithSolidColor):
        * platform/graphics/cairo/GraphicsContextCairo.cpp:
        (WebCore::GraphicsContext::setPlatformCompositeOperation):
        * platform/graphics/cg/GraphicsContextCG.cpp:
        (WebCore::GraphicsContext::setPlatformCompositeOperation):
        * platform/graphics/haiku/GraphicsContextHaiku.cpp:
        (WebCore::GraphicsContext::setPlatformCompositeOperation):
        * platform/graphics/mac/GraphicsContextMac.mm:
        (WebCore::GraphicsContext::setPlatformCompositeOperation):
        * platform/graphics/openvg/GraphicsContextOpenVG.cpp:
        (WebCore::GraphicsContext::setPlatformCompositeOperation):
        * platform/graphics/qt/GraphicsContextQt.cpp:
        (WebCore::GraphicsContext::setPlatformCompositeOperation):
        * platform/graphics/skia/GraphicsContextSkia.cpp:
        (WebCore::GraphicsContext::setPlatformCompositeOperation):
        * platform/graphics/skia/SkiaUtils.cpp:
        (WebCore::WebCoreCompositeToSkiaComposite):
        * platform/graphics/wince/GraphicsContextWinCE.cpp:
        (WebCore::GraphicsContext::setPlatformCompositeOperation):
        * platform/graphics/wx/GraphicsContextWx.cpp:
        (WebCore::GraphicsContext::setPlatformCompositeOperation):
        * rendering/RenderBoxModelObject.cpp:
        (WebCore::RenderBoxModelObject::paintFillLayerExtended):
        * rendering/RenderView.cpp:
        (WebCore::RenderView::paintBoxDecorations):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@73729 268f45cc-cd09-0410-ab3c-d52691b4dbfc