Created by: depau
- add filters: vflip, zoompan, hue, colorchannelmixer
- add options for trim, overlay
- filter code made more generic and extendable
The last one has been done by adding some helper methods to base filter class, which are called in child classes as needed. I tried not to break API but I did break it for trim, in which you put setpts='PTS-STARTPTS' by default. That can be done as a separate filter, though I didn't: I kept the "setpts" keyword argument, just in case.