Reviewed by Dan Bernstein.

        Avoid creating huge compositing layers for elements that project outside the viewport
        https://bugs.webkit.org/show_bug.cgi?id=42338

        The logic that computed the bounds of compositing layers naively used the
        union of the bounds of descendant, non-composited RenderLayers, without regard
        to what is actually visible. This could result in huge layers for page with
        elements are large negative offsets, or with large negative text-indent (both
        common).

        For elements without transforms on them or in their ancestor chain, and when
        no 3d transforms or hardware-accelerated animations are used, can clip compositing
        layers to the size of the document, or based on CSS overflow and clip.

        Tests: compositing/geometry/limit-layer-bounds-clipping-ancestor.html
               compositing/geometry/limit-layer-bounds-fixed-positioned.html
               compositing/geometry/limit-layer-bounds-overflow-repaint.html
               compositing/geometry/limit-layer-bounds-positioned-transition.html
               compositing/geometry/limit-layer-bounds-positioned.html
               compositing/geometry/limit-layer-bounds-transformed-overflow.html
               compositing/geometry/limit-layer-bounds-transformed.html

        * rendering/RenderLayerBacking.cpp:
        (WebCore::enclosingOverflowClipAncestor):  Walk up the RenderLayer tree
        looking for an ancestor that has overflow, or to the root. Along the way, check for
        transformed elements.
        (WebCore::RenderLayerBacking::updateCompositedBounds):  If we're in "consult
        overlap" mode, and we don't have transforms, then constrain the bounds
        of composited layers by the RenderView's layoutOverflowRect(), or by the
        enclosing layer with overflow.
        (WebCore::RenderLayerBacking::updateGraphicsLayerGeometry): If the offset from the renderer changes,
        we need to repaint the layer.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@63452 268f45cc-cd09-0410-ab3c-d52691b4dbfc