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.h

@@ -374,9 +374,6 @@ EXPORT bool obs_source_removed(obs_source_t source);
  */
 EXPORT uint32_t obs_source_get_output_flags(obs_source_t source);
 
-/** Specifies whether the source can be configured */
-EXPORT bool obs_source_hasconfig(obs_source_t source);
-
 /** Updates settings for this source */
 EXPORT void obs_source_update(obs_source_t source, const char *settings);