Add invisible slots to computer GUIs

This ensures inventory slots are synced while the container is open,
meaning the hotbar (which is visible underneath the GUI) correctly
updates.

Fixes #915

src/main/java/dan200/computercraft/shared/command/CommandComputerCraft.java

@@ -237,7 +237,7 @@ public final class CommandComputerCraft
                         @Override
                         public Container createMenu( int id, @Nonnull PlayerInventory player, @Nonnull PlayerEntity entity )
                         {
-                            return new ContainerViewComputer( id, computer );
+                            return new ContainerViewComputer( id, player, computer );
                         }
                     } );
                     return 1;