Fix T82960: Inaccurate selection on collapsed outliner rows

After rB15083d9e1 the outliner tree is not rebuilt after expanding or
collapsing rows. Because the tree is no longer rebuilt the positions
and flags of the elements are not cleared when collapsing a row. This
caused hover highlights and selections on the collapsed child icons to
be incorrect in many cases.

For example, only the direct children of a collapsed element are drawn
inline. If any grandchild elements had been previously icon row flagged
they would continue to be evaluated as icon row elements despite being
hidden. In this case the x coordinates of the child and grandchild would
overlap causing selection to appear erratic.

Now the flags for inline row icons are explicitly cleared, which was
previously only done because the tree was rebuilt on collapsing rows.

source/blender/editors/space_outliner/outliner_draw.c

@@ -2845,6 +2845,7 @@ static void outliner_draw_iconrow(bContext *C,
 
   LISTBASE_FOREACH (TreeElement *, te, lb) {
     TreeStoreElem *tselem = TREESTORE(te);
+    te->flag &= ~(TE_ICONROW | TE_ICONROW_MERGED);
 
     /* object hierarchy always, further constrained on level */
     if (level < 1 || (tselem->type == 0 && te->idcode == ID_OB)) {