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/obs.c

@@ -41,7 +41,7 @@ static inline void make_video_info(struct video_output_info *vi,
 		struct obs_video_info *ovi)
 {
 	vi->name    = "video";
-	vi->type    = ovi->output_format;
+	vi->format  = ovi->output_format;
 	vi->fps_num = ovi->fps_num;
 	vi->fps_den = ovi->fps_den;
 	vi->width   = ovi->output_width;
@@ -381,7 +381,7 @@ bool obs_get_video_info(struct obs_video_info *ovi)
 	ovi->base_height   = video->base_height;
 	ovi->output_width  = info->width;
 	ovi->output_height = info->height;
-	ovi->output_format = info->type;
+	ovi->output_format = info->format;
 	ovi->fps_num       = info->fps_num;
 	ovi->fps_den       = info->fps_den;