Fix divide by zero bug in late game

If the game got late enough that move_importance(currentPly) * slowMover / 100
rounds to 0, then we ended up dividing 0 by 0 when only looking 1 move ahead.

This apparently caused the search to almost immediately abort and Stockfish
would blunder in long games. So convert thisMoveImportance to a double.

No functional change.
Merge branch 'master' into imbalance
Imbalance: tune bishop (take 1)

bench: 9198510
Imbalance: tune bishop (take 2)

bench: 9164767
Imbalance: tune bishop (take 3)

bench: 8626275
Imbalance: tune bishop (take 4)

bench: 9445449