Reviewed by Ariya Hidayat.

        Directly composited SVG images fail to render
        https://bugs.webkit.org/show_bug.cgi?id=51975

        There was already code, added in r67900, to detect whether an image
        may be directly composited, by checking to see if it's a bitmap image.

        However, CachedImage::image() returns the nullImage() if m_image is null,
        which is a BitmapImage, confusing this test.

        Test: compositing/images/direct-svg-image.html

        * loader/cache/CachedImage.cpp: Remove nullImage(), since Image has
        exactly the same method.
        (WebCore::CachedImage::image): Use Image::nullImage().
        * loader/cache/CachedImage.h:
        (WebCore::CachedImage::hasImage): New method to ask whether the m_image is
        non-null.
        * rendering/RenderLayerBacking.cpp:
        (WebCore::RenderLayerBacking::isDirectlyCompositedImage): Use hasImage()
        so that we don't test the nullImage.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@75137 268f45cc-cd09-0410-ab3c-d52691b4dbfc