This is mostly a preparation for reverse walks through the movement queue,
where crossing speed calculation is done only once, while actually used
speeds can be raised successively with repeated walks.
This means, modify existing code to let the lookahead algorithms
do their work. It also means to remove some unused code in
dda_lookahead.c and reordering some code to make it work with
LOOKAHEAD undefined.
Markus Hitter