Fixed bug that miter line joins are cut of early.

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.

[svg-miterlimit]: http://www.w3.org/TR/SVG/painting.html#StrokeProperties