Check OpenCV performance hit

OpenCV plugins were disabled by default in MLT because is seems related to major slowdowns related to multithreading: https://github.com/mltframework/mlt/issues/274

This should be investigated since there are some nice OpenCV plugins (object tracker) and exciting development possibilities (deep learning object recognition and mask, etc)

One way to test would be to create an AppImage version with OpenCV module, and one without and compare timeline and rendering performance.