lavfi: replace passthrough_filter_frame with a flag.

With the introduction of AVFilterContext->is_disabled, we can simplify
the custom passthrough mode in filters.

This commit is technically a small compat break, but the timeline was
introduced very recently.

Doxy by Stefano Sabatini.

libavfilter/vf_removelogo.c

@@ -578,5 +578,5 @@ AVFilter avfilter_vf_removelogo = {
     .inputs        = removelogo_inputs,
     .outputs       = removelogo_outputs,
     .priv_class    = &removelogo_class,
-    .flags         = AVFILTER_FLAG_SUPPORT_TIMELINE,
+    .flags         = AVFILTER_FLAG_SUPPORT_TIMELINE_GENERIC,
 };