Let target-square highlighting prevail over legality test

Let target-square highlighting prevail over legality test

For engine-defined variants the highlight command is accepted even
with legality testing on. If an engine explicitly marks a square as a
legal destination for the picked-up piece, XBoard now always accepts
the move, even when according to its own move generation, even when based
on the engine's supplied 'piece' commands, going there would be illegal,
and legality testing is on.