Allow keyint_min to codec_context

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Overview

I would like to lobby for support of keyint_min in codec_context. This should be a straight-forward add and would allow us to fully control keyframeing behavior from python <3.

Currently we can already control the maximum keyframe interval (codec_context.gop_size) and whether to generate open/closed GOPs (codec_context.closed_gop). What is missing is to be able to set the minimum interval to complete the set :)

Existing FFmpeg API

https://ffmpeg.org/doxygen/trunk/structAVCodecContext.html#a3f920af17b8b15cc9d9465ecb732afcb

Expected PyAV API

Just like it does with all the other codec_context flags.

Example:

# force keyframes exactly every 10 frames
stream = container.add_stream("libx264", 24, ...)
stream.keyint_min = 10  # new
stream.gop_size = 10
stream.closed_gop = True

Investigation

There is no problem. Just a missing keyint_min = flags.flag_property('KEYINT_MIN') in context.pyx (I think).