Bugfix: frae grabber should be called in Mixer

Grab frames in session update fails with SessionSource and SessionGroups.

FrameGrabber.h

@@ -80,7 +80,7 @@ protected:
  */
 class FrameGrabbing
 {
-    friend class Session;
+    friend class Mixer;
 
     // Private Constructor
     FrameGrabbing();

Mixer.cpp

@@ -209,6 +209,9 @@ void Mixer::update()
     // update session and associated sources
     session_->update(dt_);
 
+    // grab frames to recorders & streamers
+    FrameGrabbing::manager().grabFrame(session_->frame(), dt_);
+
     // delete sources which failed update (one by one)
     Source *failure = session()->failedSource();
     if (failure != nullptr) {

Session.cpp

@@ -99,9 +99,6 @@ void Session::update(float dt)
     // draw render view in Frame Buffer
     render_.draw();
 
-    // grab frames to recorders & streamers
-    FrameGrabbing::manager().grabFrame(render_.frame(), dt);
-
 }