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)

plugins/obs-ffmpeg/obs-ffmpeg.c

@@ -0,0 +1,14 @@
+#include <string.h>
+#include <util/c99defs.h>
+
+EXPORT const char *enum_outputs(size_t idx);
+
+static const char *outputs[] = {"obs_ffmpeg"};
+
+const char *enum_outputs(size_t idx)
+{
+	if (idx >= sizeof(outputs)/sizeof(const char*))
+		return NULL;
+
+	return outputs[idx];
+}