Add preliminary ffmpeg plugin (still testing)

- Added some code for FFmpeg output that I'm still playing around with.
  Right now I'm just trying to get it to output to file and try to
  understand the FFmpeg/libav APIs.  Hopefully in the future this plugin
  can be used for any sort of output to FFmpeg.

- Fixed a cast warning in audio-io.c with size_t -> uint32_t

- Renamed the 'video_info' and 'audio_info' structures to
  'video_conver_info' and 'audio_convert_info' to better represent their
  actual purpose, and to avoid confusion with 'audio_output_info' and
  'video_output_info' structures.

- Removed a few macros from obs-def.h that were at one point going to be
  used but no longer going to be used (at least for now)

libobs/media-io/audio-io.h

@@ -71,7 +71,7 @@ struct audio_output_info {
 	uint64_t            buffer_ms;
 };
 
-struct audio_info {
+struct audio_convert_info {
 	uint32_t            samples_per_sec;
 	enum audio_format   format;
 	enum speaker_layout speakers;
@@ -124,7 +124,8 @@ static inline size_t get_audio_size(enum audio_format type,
 EXPORT int audio_output_open(audio_t *audio, struct audio_output_info *info);
 EXPORT void audio_output_close(audio_t audio);
 
-EXPORT void audio_output_connect(audio_t video, struct audio_info *format,
+EXPORT void audio_output_connect(audio_t video,
+		struct audio_convert_info *conversion,
 		void (*callback)(void *param, const struct audio_data *data),
 		void *param);
 EXPORT void audio_output_disconnect(audio_t video,