Fix transparency bounding box

[tonymarklove]

The bounding box covering the transparency "graphics state" should now match the bounding box of the SVG element it is applied to. We also need to add the width of the stroke, since this isn't normally covered by the bounding box.

I was trying to be conservative previously by using prawn.state.page.dimensions to bound the whole page, but this doesn't always work. If the SVG has been scaled the page dimensions my only cover some of the coordinate space covered by the SVG.

Hopefully this is now correct. Prawn is a little bit inconsistent with how it handles bounding boxes and margins compared to other transformations. I had to use prawn.bounds.anchor to get the bounding box offset. So hopefully I'm not missing any other subtleties.

[mogest]

Thanks again @tonymarklove, a pleasure to read your code as always!