skins2: fix mute behaviour

The skins2 code checked for the audio volume to be zero. However the audio is muted with by caling
the function aout_SetMute() or aout_ToggleMute(). Use aout_IsMuted() to test for muted audio.
(cherry picked from commit 4f96ae4a)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/skins2/src/vlcproc.cpp

@@ -727,7 +727,8 @@ void VlcProc::on_volume_changed( vlc_object_t* p_obj, vlc_value_t newVal )
 
     audio_volume_t volume = aout_VolumeGet( pPlaylist );
     SET_VOLUME( m_cVarVolume, volume, false );
-    SET_BOOL( m_cVarMute, volume == 0 );
+    bool b_is_muted = aout_IsMuted( VLC_OBJECT(pPlaylist) );
+    SET_BOOL( m_cVarMute, b_is_muted );
 }
 
 void VlcProc::on_audio_filter_changed( vlc_object_t* p_obj, vlc_value_t newVal )
@@ -831,7 +832,8 @@ void VlcProc::init_variables()
 
     audio_volume_t volume = aout_VolumeGet( pPlaylist );
     SET_VOLUME( m_cVarVolume, volume, false );
-    SET_BOOL( m_cVarMute, volume == 0 );
+    bool b_is_muted = aout_IsMuted( VLC_OBJECT(pPlaylist) );
+    SET_BOOL( m_cVarMute, b_is_muted );
 
     update_equalizer();
 }