dec_sub: introduce sub_control(), use it for sub_step

This means the direct libass usage can be removed from command.c, and no
weird hacks for retrieving the ASS_Track are needed.

Also fix a bug when using this feature with ordered chapters.

1 files changed, 1 insertions, 0 deletions

diff --git a/sub/sd.h b/sub/sd.h
index 4268137921..3b83197989 100644
--- a/sub/sd.h
+++ b/sub/sd.h
@@ -54,6 +54,7 @@ struct sd_functions {
     void (*uninit)(struct sd *sd);
 
     void (*fix_events)(struct sd *sd);
+    int (*control)(struct sd *sd, enum sd_ctrl cmd, void *arg);
 
     // decoder
     void (*get_bitmaps)(struct sd *sd, struct mp_osd_res dim, double pts,