Accept existing Vignette URLs

Accept existing Vignette URLs for getThumbURL(), in order to change the
mode of an existing thumbnail or convert a full-sized image to a
thumbnail.
How about
```
if (this.isLegacyUrl(url)) {
	urlParameters = this.getParametersFromLegacyUrl(url);
	url = this.createThumbnailUrl(urlParameters, sizing);
} else if (this.isThumbnailerUrl(url)) {
	urlParameters = this.getParametersFromUrl(url);
	url = this.createThumbnailUrl(urlParameters, sizing);
}
```
It would be more consistent and clear what's going on.