2011-07-22 Simon Fraser <simon.fraser@apple.com>

2011-07-22  Simon Fraser  <simon.fraser@apple.com>

        Avoiding painting backgrounds if they are fully obscures by an object's foreground
        https://bugs.webkit.org/show_bug.cgi?id=65030

        Reviewed by Dan Bernstein.

        Some pages use animated loading GIFs as the background of <img>,
        but WebKit keeps animating these after the <img> has loaded.

        Thwart this by avoiding the painting of such backgrounds, if we can
        determine that they are completely obscured by the border and content
        of the element.

        * platform/graphics/BitmapImage.h:
        (WebCore::BitmapImage::currentFrameHasAlpha): Utility method, since currentFrame()
        is protected.
        * rendering/RenderBox.cpp:
        (WebCore::RenderBox::paintBoxDecorations): Call paintBackground().
        (WebCore::RenderBox::paintBackground): New wrapper for the paintFillLayers() which
        paints the background layers, plus some code we call in a couple of places. This
        checks the new backgroundIsObscured() method before doing any painting.
        * rendering/RenderBox.h:
        (WebCore::RenderBox::backgroundIsObscured): New virtual method that determines
        whether any of the background is visible.
        * rendering/RenderBoxModelObject.h:
        * rendering/RenderBoxModelObject.cpp:
        (WebCore::BorderEdge::obscuresBackground): Returns true if this edge will
        entirely hide the background under it.
        (WebCore::RenderBoxModelObject::borderObscuresBackground): Determine whether
        the border hides the background.
        * rendering/RenderImage.cpp:
        (WebCore::RenderImage::backgroundIsObscured): Override the RenderBox method
        and return true if the image is a loaded, opaque bitmap image, and the background
        won't show in the border or padding areas.
        * rendering/RenderImage.h:
        * rendering/RenderTable.cpp:
        (WebCore::RenderTable::paintBoxDecorations): Use paintBackground().

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@91628 268f45cc-cd09-0410-ab3c-d52691b4dbfc