Turns out the libavcodec version numbers are not linear;  the feature
was added in FFmpeg 3.1 commit 998e1b8, libavcodec 57.14.0; but FFmpeg
3.0 has version 57.48.101.  Ouch!

Was accidently added by commit cafc266e

This commit suppresses the remaining deprecation warnings with FFmpeg 3.1.

The AVCodecContext attribute is deprecated.

Preparing for FFmpeg 3.1 support.

Improve initial seek by not reading/decoding the first frame before
checking for the seek command.

Use the duration of the stream we're actually decoding - not the
"global" attribute AVFormatContext::duration which may differ.

Revert commit 70495aad by rewriting it.  Turns out, in old FFmpeg
versions, copying the AVPacket is necessary.

Reduce some overhead.  It is not necessary to copy the object.

av_free_packet() was deprecated in FFmpeg 3.0.