libass: add message callback

Add a message callback function to get libass messages into VLC's
logging system.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/libass.c

@@ -120,6 +120,23 @@ static void RegionDraw( subpicture_region_t *p_region, ASS_Image *p_img );
 
 //#define DEBUG_REGION
 
+static void MessageCallback( int level, const char *fmt, va_list args, void *data )
+{
+    decoder_t *p_dec = (decoder_t *)data;
+
+    // translate verbosity level
+    if( level > 6 )
+        return;
+    else if( level > 4 )
+        level = VLC_MSG_DBG;
+    else if( level > 1 )
+        level = VLC_MSG_WARN;
+    else
+        level = VLC_MSG_ERR;
+
+    msg_GenericVa( p_dec, level, MODULE_STRING, fmt, args );
+}
+
 /*****************************************************************************
  * Create: Open libass decoder.
  *****************************************************************************/
@@ -155,6 +172,9 @@ static int Create( vlc_object_t *p_this )
         return VLC_EGENERIC;
     }
 
+    /* Add message callback for logging */
+    ass_set_message_cb( p_sys->p_library, MessageCallback, p_dec );
+
     /* load attachments */
     input_attachment_t  **pp_attachments;
     int                   i_attachments;