This modifies the in-search TB probing to continue searching for a mate "behind"...

This modifies the in-search TB probing to continue searching for a mate "behind" a TB win (or loss). (#1285)

It does the following:

- If a TB win or loss value allows an alpha or beta cutoff, the cutoff is taken.
- Otherwise, the search of the current subtree continues. In PV nodes, the final value returned is adjusted to reflect that the position is a TB win (or loss).

The patch also fixes a potential problem caused by root_probe() and root_probe_wdl() dirtying the root-move scores.

This patch removes the limitation of current master that a mate is never found if the root position is not yet in the TBs, but the path to mate at some point enters the TBs. The patch is intended to preserve the efficiency and effectiveness of the current TB probing approach.

No functional change (withouth TB)