Fixes for using FFmpeg 6.1 due to deprecations. Uses `#if` macros to
allow builds for using older versions of FFmpeg.
AVFrame.key_frame was replaced with a flag in AVFrame.flags. The commit
adding the flag is [1] in FFmpeg's repository, and the deprecation is in
commit [2].
In summary of the "key_frame" change, AVFrame.key_frame is deprecated,
and AVFrame.flags indicates with a bit flag if it is a key frame (with
the enum/defined AV_FRAME_FLAG_KEY).
[1]: avutil/frame: add a keyframe flag to AVFrame
cc11191fda
[2]: avutil/frame: deprecate key_frame
3e06f6f040
6e080a6806