Add font-size to root <svg> to support proper rem sizing

I noticed an issue that the element and font sizing in the rasterized version of some pages is oddly different from the displayed page.  After a bit of investigation, I found that the size of elements with properties specified in [CSS3 rem units](http://www.w3.org/TR/css3-values/#rem-unit) is determined based on the font-size of the root element which, when appearing in an SVG document, this is the root `<svg>` element rather than the `<html>` element which appears inside `<foreignObject>`.

To support such pages, this pull request attempts to copy the font-size from the HTML document root to the SVG document root using `getComputedStyle` with fallback to `currentStyle` and `style` if neither of the others work (to support detached documents, particularly for testing).

I'd appreciate your thoughts on it.  Thanks,
Kevin