Future-proof libav/FFmpeg usage

For one thing, there are quite a few deprecation warnings still:

../../modules/codec/avcodec/subtitle.c:264:13: warning: ‘pict’ is deprecated [-Wdeprecated-declarations]
../../modules/codec/avcodec/subtitle.c:264:13: warning: ‘data’ is deprecated [-Wdeprecated-declarations]
../../modules/codec/avcodec/subtitle.c:268:13: warning: ‘pict’ is deprecated [-Wdeprecated-declarations]
../../modules/codec/avcodec/subtitle.c:268:13: warning: ‘data’ is deprecated [-Wdeprecated-declarations]
../../modules/codec/avcodec/audio.c:222:5: warning: ‘refcounted_frames’ is deprecated [-Wdeprecated-declarations]
     attribute_deprecated int refcounted_frames;
../../modules/codec/avcodec/video.c:526:5: warning: ‘refcounted_frames’ is deprecated [-Wdeprecated-declarations]
     attribute_deprecated int refcounted_frames;
../../modules/codec/avcodec/vaapi.c:58:12: warning: ‘vaapi_context’ is deprecated [-Wdeprecated-declarations]
 struct attribute_deprecated vaapi_context {
../../modules/codec/avcodec/vaapi.c:58:12: warning: ‘vaapi_context’ is deprecated [-Wdeprecated-declarations]
 struct attribute_deprecated vaapi_context {
../../modules/codec/avcodec/vaapi.c:58:12: warning: ‘vaapi_context’ is deprecated [-Wdeprecated-declarations]
 struct attribute_deprecated vaapi_context {
../../modules/codec/avcodec/encoder.c:1168:13: warning: ‘noise_reduction’ is deprecated [-Wdeprecated-declarations]
../../modules/codec/avcodec/encoder.c:1176:13: warning: ‘noise_reduction’ is deprecated [-Wdeprecated-declarations]
../../modules/codec/avcodec/encoder.c:1287:8: warning: ‘coded_frame’ is deprecated [-Wdeprecated-declarations]
     attribute_deprecated AVFrame *coded_frame;
../../modules/demux/avformat/demux.c:425:13: warning: ‘codec’ is deprecated [-Wdeprecated-declarations]
../../modules/demux/avformat/demux.c:426:13: warning: ‘codec’ is deprecated [-Wdeprecated-declarations]
../../modules/demux/avformat/demux.c:426:13: warning: ‘codec’ is deprecated [-Wdeprecated-declarations]
../../modules/demux/avformat/demux.c:426:13: warning: ‘codec’ is deprecated [-Wdeprecated-declarations]
../../modules/demux/avformat/mux.c:239:5: warning: ‘codec’ is deprecated [-Wdeprecated-declarations]

(There may be more with more bleeding-edge upstream even.)

Those things constitute a proverbial ticking time-bomb. VLC 3.0 will eventually need to be built with newer libav/FFmpeg, e.g. for regression bisection, or for stable tree continuous integration on Debian.

We can't predict all future breakages. But we can at least predict those listed above...