Cleanup: VSE effect relationship checking

Use `SEQ_relation_is_effect_of_strip` for checking effect-input
relationship where this is applicable.

source/blender/sequencer/intern/iterator.c

@@ -20,6 +20,7 @@
 #include "BKE_scene.h"
 
 #include "SEQ_iterator.h"
+#include "SEQ_relations.h"
 #include "SEQ_render.h"
 #include "SEQ_time.h"
 #include "render.h"
@@ -241,15 +242,6 @@ static void collection_filter_channel_up_to_incl(SeqCollection *collection, cons
   }
 }
 
-static bool seq_is_effect_of(const Sequence *seq_effect, const Sequence *possibly_input)
-{
-  if (seq_effect->seq1 == possibly_input || seq_effect->seq2 == possibly_input ||
-      seq_effect->seq3 == possibly_input) {
-    return true;
-  }
-  return false;
-}
-
 /* Check if seq must be rendered. This depends on whole stack in some cases, not only seq itself.
  * Order of applying these conditions is important. */
 static bool must_render_strip(const Sequence *seq, SeqCollection *strips_at_timeline_frame)
@@ -262,7 +254,8 @@ static bool must_render_strip(const Sequence *seq, SeqCollection *strips_at_time
       return false;
     }
 
-    if ((seq_iter->type & SEQ_TYPE_EFFECT) != 0 && seq_is_effect_of(seq_iter, seq)) {
+    if ((seq_iter->type & SEQ_TYPE_EFFECT) != 0 &&
+        SEQ_relation_is_effect_of_strip(seq_iter, seq)) {
       /* Strips in same channel or higher than its effect are rendered. */
       if (seq->machine >= seq_iter->machine) {
         return true;

source/blender/sequencer/intern/strip_edit.c

@@ -154,8 +154,7 @@ static void sequencer_flag_users_for_removal(Scene *scene, ListBase *seqbase, Se
     }
 
     /* Remove effects, that use seq. */
-    if ((user_seq->seq1 && user_seq->seq1 == seq) || (user_seq->seq2 && user_seq->seq2 == seq) ||
-        (user_seq->seq3 && user_seq->seq3 == seq)) {
+    if (SEQ_relation_is_effect_of_strip(user_seq, seq)) {
       user_seq->flag |= SEQ_FLAG_DELETE;
       /* Strips can be used as mask even if not in same seqbase. */
       sequencer_flag_users_for_removal(scene, &scene->ed->seqbase, user_seq);

source/blender/sequencer/intern/strip_relations.c

@@ -42,7 +42,7 @@ bool SEQ_relation_is_effect_of_strip(const Sequence *effect, const Sequence *inp
 /* check whether sequence cur depends on seq */
 static bool seq_relations_check_depend(Sequence *seq, Sequence *cur)
 {
-  if (cur->seq1 == seq || cur->seq2 == seq || cur->seq3 == seq) {
+  if (SEQ_relation_is_effect_of_strip(cur, seq)) {
     return true;
   }