Fix Inventory#setItemAmount return value ignoring max stack size if no events called

Author: NichtStudioCode

invui/src/main/java/xyz/xenondevs/invui/inventory/Inventory.java

@@ -965,7 +965,7 @@ public int setItemAmount(@Nullable UpdateReason updateReason, int slot, int amou
         } else {
             setDirectBackingItem(slot, newItemStack); // already cloned above
             notifyWindows(slot);
-            return amount;
+            return newItemStack != null ? newItemStack.getAmount() : 0;
         }
 
         return currentStack.getAmount();