Support export as mjpeg

mjpeg is a video format which basically consists of an array of jpegs. An encoder for should be fairly easy to write in Dart. There are a couple of implementations in various other languages which can be used as a starting point.

- https://github.com/jcodec/jcodec/tree/master/src/main/java/org/jcodec/codecs/mjpeg
- https://github.com/lessthanoptimal/BoofCV/blob/v0.26/main/io/src/boofcv/io/video/CreateMJpeg.java