Pull arg parsing out of atc_renderer

Having the argument parsing in the atc_renderer code makes it much
harder to use it as a library. Pull the arg parsing into the
application.

As it turns out, atc didn't end up being used in this manner, but fix
it now to avoid this potentially needing to be addressed again in the
future.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>