Support preserveAspectRatio for image elements

Currently in SVGKit preserveAspectRatio is supported for &lt;svg&gt; elements but not for &lt;image&gt; elements. This pull request adds support for supporting preserveAspectRatio for raster images in &lt;image&gt; elements. It even supports the "slice" option which the &lt;svg&gt; support for preserveAspectRatio doesn't support.
This interface is defined by the spec. We cannot add to it.

If you need to add new methods, you'll need to create a custom class and/or extension header (that does NOT begin with the letters "SVG"; it can begin with "SVGK" instead)