This patch was inspired by c065abd which updates the accumulator, if possible, based on the accumulator of two plies back if the accumulator of the preceding ply is not available.
With this patch we look back even further in the position history in an attempt to reduce the number of complete recomputations. When we find a usable accumulator for the position N plies back, we also update the accumulator of the position N-1 plies back because that accumulator is most likely to be helpful later when evaluating positions in sibling branches. By not updating all intermediate accumulators immediately, we avoid doing too much work that is not certain to be useful. Overall, roughly 2-3% speedup.
This patch makes the code more specific to the net architecture, changing input features of the net will require additional changes to the incremental update code as discussed in the PR #3193 and #3191.
