skins2: correct volume slider stuck to mute once requested by user.

This problem appeared in vlc1.1.
setting volume from core or from the user slider must be differentiated, as sending back a zero value (mute) tells the core to really set value to zero (thus erasing the saved volume)

this patch reuses what is already done for position slider (similar issue)

modules/gui/skins2/src/vlcproc.cpp

@@ -256,7 +256,7 @@ void VlcProc::refreshAudio()
     audio_volume_t volume;
     aout_VolumeGet( getIntf()->p_sys->p_playlist, &volume );
     Volume *pVolume = (Volume*)m_cVarVolume.get();
-    pVolume->set( (double)volume * 2.0 / AOUT_VOLUME_MAX );
+    pVolume->set( (double)volume * 2.0 / AOUT_VOLUME_MAX , false );
 
     // Set the mute variable
     VarBoolImpl *pVarMute = (VarBoolImpl*)m_cVarMute.get();

modules/gui/skins2/vars/volume.cpp

@@ -41,7 +41,7 @@ Volume::Volume( intf_thread_t *pIntf ): VarPercent( pIntf )
 }
 
 
-void Volume::set( float percentage )
+void Volume::set( float percentage, bool updateVLC )
 {
     // Avoid looping forever...
     if( (int)(get() * AOUT_VOLUME_MAX) !=
@@ -49,6 +49,7 @@ void Volume::set( float percentage )
     {
         VarPercent::set( percentage );
 
+        if( updateVLC )
             aout_VolumeSet( getIntf()->p_sys->p_playlist,
                             (int)(get() * AOUT_VOLUME_MAX / 2.0) );
     }

modules/gui/skins2/vars/volume.hpp

@@ -37,7 +37,9 @@ class Volume: public VarPercent
         Volume( intf_thread_t *pIntf );
         virtual ~Volume() {}
 
-        virtual void set( float percentage );
+        virtual void set( float percentage, bool updateVLC );
+
+        virtual void set( float percentage ) { set( percentage, true ); }
 
         virtual string getAsStringPercent() const;
 };