Upgrade media decoder to a more current FFmpeg API
Right now most of the FFmpeg-based functionality in OpenMW relies on functions from FFmpeg 2.7 (2015) which is considered ancient by the modern FFmpeg codebase and much of its API is considered deprecated, causing a lot of the compilation time warnings and even a run-time warning during decoding bink videos when building and running OpenMW on an OS which has a much more recent version of FFmpeg library installed. Perhaps an upgrade to at least 3.x could be in order.