Timeline: sleep for _the rest_ of the frame.

This wasn't well tested class.

src/Timeline.cpp

@@ -42,7 +42,7 @@ void Timeline::nextFrame() {
     _previousFrameDuration = duration/1e6f;
 
     if(_previousFrameDuration < _minimalFrameTime) {
-        sleep(duration/1000);
+        sleep(_minimalFrameTime*1000 - duration/1000);
         now = high_resolution_clock::now();
         _previousFrameDuration = duration_cast<microseconds>(now-previousFrameTime).count()/1e6f;
     }