video_mixer: pass string by reference

daemon/src/video/video_mixer.cpp

@@ -40,7 +40,7 @@
 
 namespace sfl_video {
 
-VideoMixer::VideoMixer(const std::string id) :
+VideoMixer::VideoMixer(const std::string &id) :
     VideoGenerator::VideoGenerator()
     , id_(id)
     , width_(0)

daemon/src/video/video_mixer.h

@@ -49,7 +49,7 @@ class VideoMixer :
         public VideoFramePassiveReader
 {
 public:
-    VideoMixer(const std::string id_);
+    VideoMixer(const std::string &id);
     virtual ~VideoMixer();
 
     void setDimensions(int width, int height);