Set MinGOP in bitmovin preset when GopMode is fixed

[esonderegger]

We've been investigating an issue where videos encoded by bitmovin and packaged by https://github.com/kaltura/nginx-vod-module as HLS+fmp4 have been freezing while playing back in Safari.

I believe the underlying problem may be keyframes in the source media being encoded in the destination media instead of the GOP size being fixed.

The larger question, though, is what is the intention of the GopMode preset option when it comes to the options bitmovin provides.

[adamthesax]

Two other things to look into when we are in fixed GOP mode:

• Make sure we are forcing closed GOP openGop: false. I'm pretty sure this was part of the problem with the encode we saw a while back, but detecting open/closed GOP is a pain-in-the-butt.
• look into minKeyframeInterval and maxKeyframeInterval. GOP and Keyframe interval are used so interchangeably that it would be good to get the bottom to what bitmovin's differentiation is. The most notable difference from an API perspective, is GOP takes frames, while keyframe interval takes seconds. How we conform the framerate the video and the GOP math we are doing may impact if we use GOP size or Keyframe Interval.