vlc_text_style: wrap tx3g specifics in decoder

include/vlc_text_style.h

@@ -108,8 +108,6 @@ struct text_segment_t {
     char *psz_text;                   /**< text string of the segment */
     text_style_t *style;              /**< style applied to this segment */
     text_segment_t *p_next;           /**< next segment */
-
-    size_t i_size; //FIXME: substx3g specific
 };
 
 /**