DummyVideo: complete constructor
DummyVideo: refactoring readNextFrame
OutpuVideo: clean encodeFrame
VideoEssenceTransform: clean convert

* Add space.
* Test the result of sws_scale with height of dst (the src has the same
height, but according to ffmpeg documentation, the test concerns the
output).
VideoFrame: remove virtual destructor
VideoEssenceTransform: convert - remove unnecessary cast
Merge pull request #42 from cchampet/dev_clean_correctif

Clean