avfilter/vf_ssim: add timeline support

libavfilter/vf_ssim.c

@@ -294,7 +294,7 @@ static int do_ssim(FFFrameSync *fs)
     ret = ff_framesync_dualinput_get(fs, &master, &ref);
     if (ret < 0)
         return ret;
-    if (!ref)
+    if (ctx->is_disabled || !ref)
         return ff_filter_frame(ctx->outputs[0], master);
     metadata = &master->metadata;
 
@@ -518,4 +518,5 @@ AVFilter ff_vf_ssim = {
     .priv_class    = &ssim_class,
     .inputs        = ssim_inputs,
     .outputs       = ssim_outputs,
+    .flags         = AVFILTER_FLAG_SUPPORT_TIMELINE_INTERNAL,
 };