Returns the true index for a child built at a given index. Defaults to the given index, however if the delegate is ListWheelChildLoopingListDelegate, this value is the index of the true element that the delegate is looping to.
Example: ListWheelChildLoopingListDelegate is built by looping a list of length 8. Then, trueIndexOf(10) = 2 and trueIndexOf(-5) = 3.
int trueIndexOf(int index) => index;