Fix PNG download
Fix PNG download

The `viewBox` attribute isn't [min-x, min-y, max-x, max-y], it's [min-x,
min-y, width, height]. If min-x or min-y were negative, the image would
be blank. Some images may also have had the wrong aspect ratio.

rightscale/design-kit,rightscale/design-kit,rightscale/design-kit,rightscale/design-kit