Refactor transcode.c so it can actually transcode + use new capability to support mp3 streaming.

Also includes the skeleton for perhaps supporting video in the future. Adds more fine-grained
ffmpeg/libav compability checks. Dependency on libavresample/libswresample exchanged with
dependency on libavfilter, which seems more versatile.

src/main.c

@@ -50,6 +50,7 @@
 #include <event.h>
 #include <libavutil/log.h>
 #include <libavformat/avformat.h>
+#include <libavfilter/avfilter.h>
 
 #include <gcrypt.h>
 GCRY_THREAD_OPTION_PTHREAD_IMPL;
@@ -585,6 +586,7 @@ main(int argc, char **argv)
     }
 
   av_register_all();
+  avfilter_register_all();
 #if LIBAVFORMAT_VERSION_MAJOR >= 54 || (LIBAVFORMAT_VERSION_MAJOR == 53 && LIBAVFORMAT_VERSION_MINOR >= 13)
   avformat_network_init();
 #endif