Reviewed by Sam Weinig.

        ShadowBlur radius for CSS shadows is slightly too big
        https://bugs.webkit.org/show_bug.cgi?id=53660

        If we follow SVG gaussian blur for CSS shadows, we can end up rendering
        shadows that extend further than the CSS "blur radius", which results
        in the shadows being truncated.

        Fix with a small fudge factor to reduce the kernel diameter slightly
        for CSS shadows.

        Also more closely follow the algorithm described in the SVG spec
        for computing the kernel size for different diameters, and clean up
        some variable naming relating to the shadow bounds.

        * platform/graphics/ShadowBlur.cpp:
        (WebCore::ShadowBlur::blurLayerImage):
        (WebCore::ShadowBlur::drawRectShadowWithTiling):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@77492 268f45cc-cd09-0410-ab3c-d52691b4dbfc