The codec -> codecpar move was sufficiently long ago (libavformat
57.5.0 on 2016-04-11) that I think we can just get away with requiring
the new version. Let's try it.
But if someone complains, AVCodecParameters and AVCodecContext look
sufficiently similar we could probably just use one or the other based on
the version we're compiling with.
This significantly improves safety of the ffmpeg interface. The complex
ABIs aren't accessed directly from Rust. Instead, I have a small C
wrapper which uses the ffmpeg C API and the C headers at compile-time to
determine the proper ABI in the same way any C program using ffmpeg
would, so that the ABI doesn't have to be duplicated in Rust code.
I've tested with ffmpeg 2.x and ffmpeg 3.x; it seems to work properly
with both where before ffmpeg 3.x caused segfaults.
It still depends on ABI compatibility between the compiled and running
versions. C programs need this, too, and normal shared library
versioning practices provide this guarantee. But log both versions on
startup for diagnosing problems with this.
Fixes#7
