2017-02-09  Miguel Gomez  <magomez@igalia.com>, 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!).

        Due to a limitation of the pixman backend, which uses 16 bits to hold signed integers, cairo is

        not able to draw anything when using transformation matrices with values bigger than 32768. When

        drawing patterns into large pages, the matrices values can overflow those 16 bits, so cairo doesn't

        draw anything in, which causes the reported transparent backgrounds.

        The patch modifies the transformation matrices both from the current context and the pattern we

        are painting, to avoid them to hold values that cannot stored in 16 bits.

        There's still the possibility that this happens, but it would require using a pattern with a size

        bigger than 32768.

        No new tests.

        * platform/graphics/cairo/CairoUtilities.cpp:

        (WebCore::drawPatternToCairoContext):

    // Cairo cannot convert a cairo_matrix to a Pixman's matrix if any of its components is bigger than maximum int size of Pixman.

    // With this condition, Cairo gives up to draw given pattern entirely. To workaround this problem, we reduce the coordinate

    // space by translating CTM and destRect.

    cairo_matrix_t ctm;

    cairo_get_matrix(cr, &ctm);

    double dx = 0, dy = 0;

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

    double xScale = 1, yScale = 1;

    cairo_matrix_transform_distance(&ctm, &xScale, &yScale);

    dx = std::trunc(-dx / tileRect.width()) * tileRect.width() / xScale;

    dy = std::trunc(-dy / tileRect.height()) * tileRect.height() / yScale;

    cairo_translate(cr, dx, dy);

    FloatRect adjustedDestRect(destRect);

    adjustedDestRect.move(-dx, -dy);

    // Again, we need to reduce the coordinate of the transformation matrix we are using for the pattern.