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.
I think this is a simple merge with the latest commit...

NB: latest commit renames SVGImage/SVGImageRef to more accurate names. SVGImage isn't allowed by the spec, I believe.

But in the end, latest commit is simply saying "if that SVGImageRef fails to load ... do something different". So I think your new code should insert easily/obviously, it's just all contained in braces for "if the image loaded OK" part.

?