The user manual specifies "encoder", which is consistent with other
output plugins.  "encoding" should be deprecated.

The ScopeExit library uses C++11 initializers, which gcc 4.6 does not
support.  Let's kill support for this ancient incomplete C++11
compiler, nobody should be using it anymore.

This commit suppresses the remaining deprecation warnings with FFmpeg 3.1.

Similar to boost::scope_exit, but fewer include dependencies.

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.