Add statistics functions to outputs

Total bytes, total frames, and frames dropped.  Total frames is
generated automatically, but total bytes and total dropped frames are
returned via callbacks.

libobs/obs-output.h

@@ -52,6 +52,10 @@ struct obs_output_info {
 	obs_properties_t (*properties)(void);
 
 	void (*pause)(void *data);
+
+	uint64_t (*total_bytes)(void *data);
+
+	int (*dropped_frames)(void *data);
 };
 
 EXPORT void obs_register_output_s(const struct obs_output_info *info,