img: Generate png format thumbnails for svg images.

img: Generate png format thumbnails for svg images.

Imagemagick does not generate svg images very well, but it can convert
them to png quite well.

For browsers that don't yet support displaying svg, this also provides a
workaround; just scale the svg down to get a png. But the workaround is
partial, since scaling the image larger, or leaving it the same size will
cause the original svg to be displayed. Since browsers are actively
improving svg support, this is good enough for me.