Reviewed by Dan Bernstein.

        WebGL canvas paints background color twice
        https://bugs.webkit.org/show_bug.cgi?id=33361

        Clean up some logic around "directly composited" content: some images, video, and WebGL.

        We previously set the GraphicsLayer background color to the CSS background color
        in some cases where the box has no other decorations. However, the content
        layer's bounds do not correspond with the background box in many cases,
        and we could end up both painting the background color, and setting it on the layer.

        Simplify this logic to never use layer background colors, and thus skip allocating
        backing store only when the element has no visible box decorations.

        Tests: compositing/images/direct-image-background-color.html
               compositing/reflections/simple-composited-reflections.html
               compositing/video/video-background-color.html
               compositing/webgl/webgl-background-color.html
               compositing/webgl/webgl-blending.html

        * rendering/RenderLayerBacking.h:
        * rendering/RenderLayerBacking.cpp:
        (WebCore::is3DCanvas): Utility to determine if a RenderObject is a canvas with WebGL
        (WebCore::RenderLayerBacking::RenderLayerBacking): Remove m_hasDirectlyCompositedContent.
        (WebCore::RenderLayerBacking::updateGraphicsLayerConfiguration): canUseDirectCompositing()
        is renamed to isDirectlyCompositedImage() and only applies to images now.
        No longer set layer background color.
        (WebCore::RenderLayerBacking::updateGraphicsLayerGeometry): Just use hasPaintedContent()
        as the argument to setDrawsContent().
        (WebCore::hasBoxDecorationsOrBackground): Renamed.
        (WebCore::RenderLayerBacking::hasNonCompositingContent):
        (WebCore::hasBoxDecorationsOrBackgroundImage): Renamed from hasBoxDecorations().
        (WebCore::RenderLayerBacking::isSimpleContainerCompositingLayer): Moved test for
        hasOverflowControls() into hasNonCompositingContent() and removed the FIXME comment.
        (WebCore::RenderLayerBacking::containsPaintedContent): New method that decides
        if the layer (and its non-composited children) has any content that must be painted into backing store.
        (WebCore::RenderLayerBacking::isDirectlyCompositedImage): Renamed from canUseDirectCompositing()
        and reduced to only apply to images.
        (WebCore::RenderLayerBacking::rendererContentChanged): 3D canvas no longer falls under the
        canUseDirectCompositing() scope.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@53034 268f45cc-cd09-0410-ab3c-d52691b4dbfc