libobs: Add 'initialize' callback to services

The 'initialize' callback is used before the encoders/output start up so
it can adjust encoder settings to required values if needed.

Also added the function 'obs_encoder_active' that returns true or false
depending on whether that encoder is active or not.

libobs/obs.h

@@ -959,6 +959,9 @@ EXPORT video_t obs_encoder_video(obs_encoder_t encoder);
  */
 EXPORT audio_t obs_encoder_audio(obs_encoder_t encoder);
 
+/** Returns true if encoder is active, false otherwise */
+EXPORT bool obs_encoder_active(obs_encoder_t encoder);
+
 /** Duplicates an encoder packet */
 EXPORT void obs_duplicate_encoder_packet(struct encoder_packet *dst,
 		const struct encoder_packet *src);