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
