Fix scaling with negative viewBox origin.

This fixes translation problems when trying to scale an SVG via its width and height attributes when the viewbox's origin is negative.

Here are a couple of example SVG's:

http://www.thenounproject.com.s3.amazonaws.com/img/wide.svg
http://www.thenounproject.com.s3.amazonaws.com/img/tall.svg

And the relevant cairo commands:

`cairosvg wide.svg -f png -o wide.png`
`cairosvg tall.svg -f png -o tall.png`