change freeing of frame_mappers allocated in Timeline

- each class is now responsible to free whatever it allocates
- all tests passed on my machine with ffmpeg 3.2
- Clip is now more careful about freeing a reader if it allocated it as well

src/FrameMapper.cpp

@@ -62,7 +62,6 @@ FrameMapper::~FrameMapper() {
 		// Auto Close if not already
 		Close();
 
-	delete reader;
 	reader = NULL;
 }