lavfi: add timeline support in {de,remove}logo.

Both don't change anything in the context between frames.

libavfilter/vf_delogo.c

@@ -269,4 +269,5 @@ AVFilter avfilter_vf_delogo = {
 
     .inputs    = avfilter_vf_delogo_inputs,
     .outputs   = avfilter_vf_delogo_outputs,
+    .flags     = AVFILTER_FLAG_SUPPORT_TIMELINE,
 };

libavfilter/vf_removelogo.c

@@ -578,4 +578,5 @@ AVFilter avfilter_vf_removelogo = {
     .inputs        = removelogo_inputs,
     .outputs       = removelogo_outputs,
     .priv_class    = &removelogo_class,
+    .flags         = AVFILTER_FLAG_SUPPORT_TIMELINE,
 };