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-defs.h

@@ -31,6 +31,3 @@
 #define SOURCE_ASYNC_VIDEO    (1<<2) /* Async video (use with SOURCE_VIDEO) */
 #define SOURCE_DEFAULT_EFFECT (1<<3) /* Source uses default/filter effect */
 #define SOURCE_YUV            (1<<4) /* Source is in YUV color space */
-
-#define OUTPUT_VIDEO_ENCODER (1<<0) /* Output requires a video encoder */
-#define OUTPUT_AUDIO_ENCODER (1<<1) /* Ouptut requires an audio encoder */