Cleanup: VSE effect relationship checking

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

source/blender/editors/space_sequencer/sequencer_select.c

@@ -27,6 +27,7 @@
 
 #include "SEQ_channels.h"
 #include "SEQ_iterator.h"
+#include "SEQ_relations.h"
 #include "SEQ_select.h"
 #include "SEQ_sequencer.h"
 #include "SEQ_time.h"
@@ -1920,7 +1921,7 @@ static bool select_grouped_effect(SeqCollection *strips,
   Sequence *seq;
   SEQ_ITERATOR_FOREACH (seq, strips) {
     if (SEQ_CHANNEL_CHECK(seq, channel) && (seq->type & SEQ_TYPE_EFFECT) &&
-        ELEM(actseq, seq->seq1, seq->seq2, seq->seq3)) {
+        SEQ_relation_is_effect_of_strip(seq, actseq) {
       effects[seq->type] = true;
     }
   }

source/blender/editors/transform/transform_snap_sequencer.c

@@ -21,6 +21,7 @@
 #include "SEQ_channels.h"
 #include "SEQ_effects.h"
 #include "SEQ_iterator.h"
+#include "SEQ_relations.h"
 #include "SEQ_render.h"
 #include "SEQ_sequencer.h"
 
@@ -102,8 +103,7 @@ static void query_strip_effects_fn(Sequence *seq_reference,
 
   /* Find all strips connected to `seq_reference`. */
   LISTBASE_FOREACH (Sequence *, seq_test, seqbase) {
-    if (seq_test->seq1 == seq_reference || seq_test->seq2 == seq_reference ||
-        seq_test->seq3 == seq_reference) {
+    if (SEQ_relation_is_effect_of_strip(seq_test, seq_reference)) {
       query_strip_effects_fn(seq_test, seqbase, collection);
     }
   }

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;
   }