Produces empty SVG if stdin is closed

If stdin is closed termtosvg will create a (mostly) empty svg. E.g. running this:

     termtosvg -g 82x19 -c 'echo hello' out.svg

with closed stdin won't produce any frames. Instead one has to do something like this:

     (tail -f /dev/null || true) | termtosvg -g 82x19 -c 'echo hello' out.svg

which will produce an SVG file where we see "hello" being printed.