January 19th 2020
Modify the current state index and update the buffer contents to reflect the contents of the buffer as it was at that index.
the standard custom layer context pointer
the id of the buffer to modify
the new current state index value
non-zero if the buffer exists, has history enabled, and contains a record at the given index, otherwise zero
This call simultaneously changes the state index and modifies the buffer to reflect undoing the necessary records from the top of the stack, or redoing them if the state index is being moved forward.