Bugfix: frae grabber should be called in Mixer

Grab frames in session update fails with SessionSource and SessionGroups.

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);
-
 }