Handle Slot tag inside CONTAINER tag in 1.20.3->1.20.5 (#3995)

Fixes shulker box tooltips not showing all items for 1.20.3 clients on 1.20.5+ servers

common/src/main/java/com/viaversion/viaversion/protocols/v1_20_3to1_20_5/rewriter/StructuredDataConverter.java

@@ -18,6 +18,7 @@
 package com.viaversion.viaversion.protocols.v1_20_3to1_20_5.rewriter;
 
 import com.google.common.base.Preconditions;
+import com.viaversion.nbt.tag.ByteTag;
 import com.viaversion.nbt.tag.CompoundTag;
 import com.viaversion.nbt.tag.FloatTag;
 import com.viaversion.nbt.tag.IntArrayTag;
@@ -798,7 +799,19 @@ public final class StructuredDataConverter {
 
     private void convertItemList(final UserConnection connection, final Item[] items, final CompoundTag tag, final String key) {
         final ListTag<CompoundTag> itemsTag = new ListTag<>(CompoundTag.class);
-        for (final Item item : items) {
+        for (int i = 0; i < items.length; i++) {
+            final Item item = items[i];
+            final CompoundTag savedItem = itemToTag(connection, item);
+            // 1.20.4 clients need the Slot to display the item correctly
+            if (backupInconvertibleData) {
+                savedItem.putByte("Slot", (byte) i);
+            }
+            itemsTag.add(savedItem);
+        }
+        tag.put(key, itemsTag);
+    }
+
+    private CompoundTag itemToTag(final UserConnection connection, final Item item) {
         final CompoundTag savedItem = new CompoundTag();
         if (item != null) {
             final String name = toMappedItemName(item.identifier());
@@ -816,10 +829,7 @@ public final class StructuredDataConverter {
         } else {
             savedItem.putString("id", "air");
         }
-
-            itemsTag.add(savedItem);
-        }
-        tag.put(key, itemsTag);
+        return savedItem;
     }
 
     private void convertEnchantments(final Enchantments data, final CompoundTag tag, final boolean storedEnchantments) {