Use --export-filename instead of the deprecated --export-png

This was suggested in voc#42 and prevents a warning when using Inkscape.

renderlib.py

@@ -98,7 +98,7 @@ def renderFrame(svg, task, outfile):
                     converted.save(filename=outfile)
     else:
         # invoke inkscape to convert the generated svg-file into a png inside the .frames-directory
-        cmd = 'inkscape --export-background=white --export-background-opacity=0 --export-width={1} --export-height={2} --export-png="{3}" --pipe 2>&1 >/dev/null'.format(task.workdir, width, height, outfile)
+        cmd = 'inkscape --export-background=white --export-background-opacity=0 --export-width={1} --export-height={2} --export-filename="{3}" --pipe 2>&1 >/dev/null'.format(task.workdir, width, height, outfile)
         errorReturn = subprocess.check_output(cmd, shell=True, universal_newlines=True, input=svg.svgstr, stderr=subprocess.STDOUT, cwd=task.workdir)
         if errorReturn != '':
             print("inkscape exitted with error\n" + errorReturn)