recalculate clippath on chart.redraw; avoid Webkit select("clippath")

[davisford]

This fixes #730

On chart.redraw() the clippath is not re-generated. I use chart.redraw() a lot in responsive layouts, instead of the more heavy-handed chart.render().

[gordonwoodhull]

Hey @davisford, there was a little bug with your implementation. The good news is that for once the stock regression test was actually useful.

It picked up the following odd bit of svg:

<defs><yearly-bubble-chart-clip id="yearly-bubble-chart-clip">
   <rect width="888" height="198" transform="translate(-0, -0)"></rect>
</yearly-bubble-chart-clip></defs>

I think that would actually work, because the clippath is always selected by its ID, but nonetheless that's a pretty weird tag name!

I also added a test that fails without your fix so this should stay fixed. Thanks!!