Though usually one would export directly to "${3}", Inkscape 1.0 had
its command line options changed by people who apparently think that
backwards compatibility is some kind of swear word: Whereas earlier
Inkscape versions would export to a file called foo.png.tmp, newer
behaviour is to ignore the user's wishes & write to foo.png.png –
unless one asks it to write to a filename with a .png extension,
Inkscape 1.0 changes the filename extension to .png each time.
As we do not know the extension of "${3}", we have to use the
extension, then rename the resulting file to the proper name;
only that way the export works with Inkscape 1.0 & earlier …
