Reviewed by Darin Adler

        https://bugs.webkit.org/show_bug.cgi?id=22981

        Repaints inside of an SVGImage were thrown on the floor,
        which broke incremental painting due to loading, or SVG animation.
        Fix this by assigning the SVGImage a ChromeClient subclass that passes
        along repaints via a new method on ImageObserver, which also takes
        a rect parameter for the changed rect, allowing incremental repaints.
        Fix RenderImage::imageChanged to take advantage of this changedRect to
        only repaint the changed parts of the image.

        This also enables incremental painting for canvas-as-image, so
        add tests for that too.

        Tests: fast/backgrounds/animated-svg-as-background.html
               fast/backgrounds/animated-svg-as-mask.html
               fast/canvas/canvas-as-image-incremental-repaint.html
               fast/canvas/canvas-as-image.html
               fast/images/animated-svg-as-image.html

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@39555 268f45cc-cd09-0410-ab3c-d52691b4dbfc