BMediaEventLooper: Make lateness calculus in real time

* After having this patch applied locally for some time,
I'm going to be sure that we have to deal only with real_time values.
* Since RealTime() is just a system_time() wrapper, the code
will be slightly improved in performances too.
* Additionally it performs very well when the thread is
externally stopped (such as debugging it), so that the audio
recover almost immediately.

src/kits/media/MediaEventLooper.cpp

@@ -241,12 +241,11 @@ BMediaEventLooper::ControlLoop()
 				err = fRealTimeQueue.RemoveFirstEvent(&event);
 
 			if (err == B_OK) {
-				// We are going to do this calculus in performance time
+				// We are going to do this calculus in real time
 				// because otherwise we could get erroneous values.
 				// This calculus allow us to detect both early and late
 				// buffers, this is the meaning of the lateness concept.
-				bigtime_t lateness = event.event_time - fEventLatency
-					- fSchedulingLatency - TimeSource()->Now();
+				bigtime_t lateness = waitUntil - TimeSource()->RealTime();
 				DispatchEvent(&event, -lateness, hasRealtime);
 			}
 		} else if (err != B_OK)