Stop Flock gnesis turret trying to fire through windows

[amylizzle]

About the PR

Basically adds "can a projectile path directly from the turret to the target" as a final check on the valid target checking. Uses jpsTurfPassable to take advantage of path caching, which will likely be heavily in use around flock structures.

Why's this needed?

Gnesis turrets kept wasting their ammo trying to shoot through windows, which made them nearly useless.

Changelog

(u)Amylizzle
(+)Flock gnesis turrets will now only shoot at targets they can hit, instead of trying to shoot through obstructions.

[TobleroneSwordfish]

lgtm, probably reasonably performant too, may be good to generalise in future for other LoS checks