Reviewed by Simon Fraser

        https://bugs.webkit.org/show_bug.cgi?id=23361

        When using hardware compositing, some images can be directly
        rendered by the hardware - no need to draw them into a separate
        context, therefore saving memory.
        Only images with certain style properties can be directly
        composited - basically anything that is not a simple image requires
        the usual rendering path (eg. if the image has borders).

        Test: compositing/direct-image-compositing.html

        * manual-tests/resources/simple_image.png: Added.
        * manual-tests/simple-image-compositing.html: Added.
        * platform/graphics/Image.h:
        (WebCore::Image::startAnimation):
            - move this to public
        * rendering/RenderImage.cpp:
        (WebCore::RenderImage::imageChanged):
            - poke compositing layer if image has changed
        (WebCore::RenderImage::notifyFinished):
            - let the compositing layer know that it can render the image
        * rendering/RenderImage.h:
        * rendering/RenderLayer.cpp:
        (WebCore::RenderLayer::rendererContentChanged):
        * rendering/RenderLayer.h:
        * rendering/RenderLayerBacking.cpp:
        (WebCore::RenderLayerBacking::canUseInnerContentLayer):
        (WebCore::RenderLayerBacking::detectDrawingOptimizations):
        (WebCore::RenderLayerBacking::rendererContentChanged):
        * rendering/RenderLayerBacking.h:
            - code to hook up direct compositing of images where
              possible


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@41996 268f45cc-cd09-0410-ab3c-d52691b4dbfc