filter_chain: filter_chain_GetFmtOut can use a const input

include/vlc_filter.h

@@ -408,7 +408,7 @@ VLC_API bool filter_chain_IsEmpty(const filter_chain_t *chain);
  *
  * \param chain filter chain
  */
-VLC_API const es_format_t *filter_chain_GetFmtOut(filter_chain_t *chain);
+VLC_API const es_format_t *filter_chain_GetFmtOut(const filter_chain_t *chain);
 
 /**
  * Apply the filter chain to a video picture.

src/misc/filter_chain.c

@@ -381,9 +381,8 @@ bool filter_chain_IsEmpty(const filter_chain_t *chain)
     return chain->first == NULL;
 }
 
-const es_format_t *filter_chain_GetFmtOut( filter_chain_t *p_chain )
+const es_format_t *filter_chain_GetFmtOut( const filter_chain_t *p_chain )
 {
-
     if( p_chain->b_allow_fmt_out_change )
         return &p_chain->fmt_out;