fix(marks): ensure decor is removed with proper range (#32973)

Problem:  Paired mark whose end is in front of its start should not have
          its decor removed (as fixed by 72f630f9), but may still need to
          have its range redrawn.
Solution: Still call `buf_decor_remove()` but ensure it is not called with
          an inverse range when `extmark_del()` is called on an end mark.

src/nvim/extmark.c

@@ -180,11 +180,14 @@ void extmark_del(buf_T *buf, MarkTreeIter *itr, MTKey key, bool restore)
   }
 
   if (mt_decor_any(key)) {
-    // If key is an end mark it has been found first while iterating the marktree,
-    // indicating the decor is already invalid.
-    if (mt_invalid(key) || mt_end(key)) {
+    if (mt_invalid(key)) {
       decor_free(mt_decor(key));
     } else {
+      if (mt_end(key)) {
+        MTKey k = key;
+        key = key2;
+        key2 = k;
+      }
       buf_decor_remove(buf, key.pos.row, key2.pos.row, key.pos.col, mt_decor(key), true);
     }
   }