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.

Don't scan tag items if the handler doesn't implement the tag()
method.