Refactor: Remove hack to store sequence type in Outliner tree element

`TreeElement.idcode` would be reused to store the sequence type. This is
risky if the field is assumed to actually contain a valid ID-code,
without further checks.
This was only accessed in one place, which I've refactored to a clean,
type-safe solution now.

source/blender/editors/space_outliner/outliner_draw.cc

@@ -72,6 +72,7 @@
 #include "tree/tree_element_id.hh"
 #include "tree/tree_element_overrides.hh"
 #include "tree/tree_element_rna.hh"
+#include "tree/tree_element_seq.hh"
 #include "tree/tree_iterator.hh"
 
 namespace blender::ed::outliner {
@@ -2762,8 +2763,9 @@ TreeElementIcon tree_element_get_icon(TreeStoreElem *tselem, TreeElement *te)
       case TSE_POSEGRP:
         data.icon = ICON_GROUP_BONE;
         break;
-      case TSE_SEQUENCE:
-        switch (te->idcode) {
+      case TSE_SEQUENCE: {
+        const TreeElementSequence *te_seq = tree_element_cast<TreeElementSequence>(te);
+        switch (te_seq->getSequenceType()) {
           case SEQ_TYPE_SCENE:
             data.icon = ICON_SCENE_DATA;
             break;
@@ -2816,6 +2818,7 @@ TreeElementIcon tree_element_get_icon(TreeStoreElem *tselem, TreeElement *te)
             break;
         }
         break;
+      }
       case TSE_SEQ_STRIP:
         data.icon = ICON_LIBRARY_DATA_DIRECT;
         break;

source/blender/editors/space_outliner/tree/tree_element_seq.cc

@@ -22,13 +22,6 @@ TreeElementSequence::TreeElementSequence(TreeElement &legacy_te, Sequence &seque
     : AbstractTreeElement(legacy_te), sequence_(sequence)
 {
   BLI_assert(legacy_te.store_elem->type == TSE_SEQUENCE);
-
-  /*
-   * The idcode is a little hack, but the outliner
-   * only check te->idcode if te->type is equal to zero,
-   * so this is "safe".
-   */
-  legacy_te.idcode = sequence_.type;
   legacy_te.name = sequence_.name + 2;
 }
 
@@ -62,6 +55,11 @@ Sequence &TreeElementSequence::getSequence() const
   return sequence_;
 }
 
+SequenceType TreeElementSequence::getSequenceType() const
+{
+  return SequenceType(sequence_.type);
+}
+
 /* -------------------------------------------------------------------- */
 /* Strip */
 
@@ -86,8 +84,6 @@ TreeElementSequenceStripDuplicate::TreeElementSequenceStripDuplicate(TreeElement
     : AbstractTreeElement(legacy_te), sequence_(sequence)
 {
   BLI_assert(legacy_te.store_elem->type == TSE_SEQUENCE_DUP);
-
-  legacy_te_.idcode = sequence.type;
   legacy_te_.name = sequence.strip->stripdata->filename;
 }
 

source/blender/editors/space_outliner/tree/tree_element_seq.hh

@@ -8,6 +8,8 @@
 
 #pragma once
 
+#include "DNA_sequence_types.h"
+
 #include "tree_element.hh"
 
 struct Sequence;
@@ -25,6 +27,7 @@ class TreeElementSequence : public AbstractTreeElement {
   void expand(SpaceOutliner &) const override;
 
   Sequence &getSequence() const;
+  SequenceType getSequenceType() const;
 };
 
 /* -------------------------------------------------------------------- */

source/blender/makesdna/DNA_sequence_types.h

@@ -679,7 +679,7 @@ enum {
  *
  * \warning #SEQ_TYPE_EFFECT BIT is used to determine if this is an effect strip!
  */
-enum {
+typedef enum SequenceType {
   SEQ_TYPE_IMAGE = 0,
   SEQ_TYPE_META = 1,
   SEQ_TYPE_SCENE = 2,
@@ -731,7 +731,7 @@ enum {
   SEQ_TYPE_EXCLUSION = 60,
 
   SEQ_TYPE_MAX = 60,
-};
+} SequenceType;
 
 enum {
   SEQ_MOVIECLIP_RENDER_UNDISTORTED = 1 << 0,