Pass absolute file paths to inkscape

Since the command is run in `task.workdir` instead of the original working
directory, the potentially relative paths passed by the caller(s) may not
point to actual files when running inkscape, which results in failed
renderings.

This seems to have mainly affected running under `--debug`, but should fix
voc#40 (which doesn't say the issue was seen under `--debug` though).

renderlib.py

@@ -93,7 +93,7 @@ def renderFrame(infile, 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}" "{4}" 2>&1 >/dev/null'.format(task.workdir, width, height, outfile, infile)
+        cmd = 'inkscape --export-background=white --export-background-opacity=0 --export-width={1} --export-height={2} --export-png="{3}" "{4}" 2>&1 >/dev/null'.format(task.workdir, width, height, os.path.abspath(outfile), os.path.abspath(infile))
         errorReturn = subprocess.check_output(cmd, shell=True, universal_newlines=True, stderr=subprocess.STDOUT, cwd=task.workdir)
         if errorReturn != '':
             print("inkscape exitted with error\n" + errorReturn)