GvcMixerStream id is unsigned int and starts with 1, so we can use
0 as invalid id. This fixes build error/warning - comparison
between signed and unsigned integer expressions.
This will allow to have different icons for internal audio cards
(which are flagged generically as "audio-card"), depending on which
port is in use (ie. headphones or speakers).
This requires the new icon information, which is only exported by
PulseAudio 3.0. If it's not available, we fallback to card icons
like before.
https://bugzilla.gnome.org/show_bug.cgi?id=689931
