Only apply clipping masks to elements that have the clip-path attribute.

Suppose you have an SVG document like:

    <g clip-path="url(#someClipPath)">
        <!-- lots of children here -->
    </g>

The code would redundantly apply the clipping path from someClipPath to the layer of every one of those children, as well as to the &lt;g&gt; layer. The result was huge slowness if there were many children. This pull request fixes it to only apply the clipping path to the layer with the clip-path attribute. Child layers will get clipped by their parent layer.