2016-02-22  Gwang Yoon Hwang  <yoon@igalia.com>

        [GTK] scroll with transparent background not repainted after scrollY >= 32768

        https://bugs.webkit.org/show_bug.cgi?id=154283

        Reviewed by NOBODY (OOPS!).

        If the Cairo's matrix exceeds the limit of Pixman's maximum int size (which is

        15bit), Cairo does not render bitmap because the given matrix is invalid.  When

        we scroll a page which has a patterned bitmap as a background to more than the

        Pixman's maximum int size, it simply doesn't render anything as background and

        the target surface ends up with a transparent background.

        This patch translates the CTM of Cairo's context when we drawing patterns to

        the context to remove invisible area. Because it is a pattern there is no

        visual difference between drawing tiles from the visible area and the beginning

        of the page. But it is more natural to draw tiles from the visible area for the

        performance.

        * platform/graphics/cairo/CairoUtilities.cpp:

        (WebCore::drawPatternToCairoContext):

    cairo_matrix_t ctm;

    double dx = 0, dy = 0;

    cairo_get_matrix(cr, &ctm);

    cairo_matrix_transform_point(&ctm, &dx, &dy);

    if (dx < 0)

        dx = std::floor(std::abs(dx / tileRect.width()));

    if (dy < 0)

        dy = std::floor(std::abs(dy / tileRect.height()));

    cairo_translate(cr, dx, dy);