This is consistent with the core Godot source code, and ensures the
license isn't detached from its original code when individual files
are distributed.
This is needed since that operator returns a local copy, not
an lvalue. Attempting to write to the return value of these operators
wouldn't change the array element. PoolVectors need locking when
writing, so this operator can't return a writable reference.
To update a Pool*Array, use the `set()` method which locks and unlocks
the array. For multiple writes, use the `write()` method which returns
a locked writable view, and unlocks when it goes out of scope.
