Reviewed by Simon Fraser.

        Accelerated 2d canvases should get compositing layers
        https://bugs.webkit.org/show_bug.cgi?id=43362

        Allows for 2d canvas rendering contexts to use a GraphicsContext3D for
        accelerated rendering.  The rendering context holds the GraphicsContext3D
        alive and exposes it to the compositor.  The 3d context itself is passed
        down through the GraphicsContext to use for actual rendering:
        https://bug-43362-attachments.webkit.org/attachment.cgi?id=63557

        This approach will let us simultaneously prototype accelerated techniques
        for canvas 2d and revise the design of canvas elements without destabilizing
        all ports.

        * html/HTMLCanvasElement.cpp:
        (WebCore::HTMLCanvasElement::getContext):
        (WebCore::HTMLCanvasElement::willDraw):
        (WebCore::HTMLCanvasElement::makeRenderingResultsAvailable):
        * html/canvas/CanvasRenderingContext.h:
        (WebCore::CanvasRenderingContext::paintRenderingResultsToCanvas):
        * html/canvas/CanvasRenderingContext2D.cpp:
        (WebCore::CanvasRenderingContext2D::CanvasRenderingContext2D):
        (WebCore::CanvasRenderingContext2D::isAccelerated):
        (WebCore::CanvasRenderingContext2D::reset):
        (WebCore::CanvasRenderingContext2D::willDraw):
        (WebCore::CanvasRenderingContext2D::paintRenderingResultsToCanvas):
        * html/canvas/CanvasRenderingContext2D.h:
        (WebCore::CanvasRenderingContext2D::graphicsContext3D):
        * html/canvas/WebGLRenderingContext.cpp:
        (WebCore::WebGLRenderingContext::paintRenderingResultsToCanvas):
        * html/canvas/WebGLRenderingContext.h:
        * platform/graphics/GraphicsContext.h:
        * platform/graphics/skia/GraphicsContextSkia.cpp:
        (WebCore::GraphicsContext::syncSoftwareCanvas):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@64881 268f45cc-cd09-0410-ab3c-d52691b4dbfc