Fixed bug that miter line joins are cut of early and one with stacked clippings.

Fixed bug that miter line joins are cut of early by explicitly setting a high miter limit.

I noticed that the [miter limit][svg-miterlimit] was left at its default value by the SVG backend while looking at the Charts drawing test for lines. I don't know if a even higher value would be good here or an actual miter limit attribute for Diagrams, but this value works for my use cases.

I also fixed a issue with stacked clippings. Before this several stacked clips were just printed as several paths in the clipPath element. But according to the [SVG spec][svg-clipping] this means they are OR'ed together, although we want them to be intersected. I changed this by building them up around the clipped element hierarchical, which leads to intersection according to the SVG spec. I tested this in the chart drawing tests and it seems to work.

[svg-miterlimit]: http://www.w3.org/TR/SVG/painting.html#StrokeProperties
[svg-clipping]: http://www.w3.org/TR/SVG/masking.html#EstablishingANewClippingPath