Turret firing arc

[Elyssaen]

Weapons can optionally have the weapon attribute "firing arc" with a
number between 0 and 180, and can swivel that many degrees left or right
of forward. This can allow for swivel cannons (can set up using a gun
mount as normal, and choose a "firing arc" that's fairly low) or for turrets
with a blind spot at the back. By default, anything taking a turret mount
will swivel the full 180, as before.

Missile boat AI will trigger somewhere between 1000 and 2000 range
depending on the swivel angle: as before, fixed guns at 2000 and normal
turrets at 1000, but e.g. a 90° swivel turret will trigger missile boat
AI at 1500 range.

This can also be used on anti-missile weapons. Lastly, if you try to fire
while outside of the firing arc, the gun will try to far as far toward the
target as it can: in practice, this is more playable than not firing at all,
or reverting to fire straight forward.

Context: though this is generally useful, I wrote it for my Trin Mobulas. They're stocky battlecruisers which rely on fixed firepower, and have serious convergence problems if they use normal fixed guns. I wanted to create some swivel cannons for them modelled on Escape Velocity Override's swivel phase cannons (used by Azdaras) and EV Nova's Fusion Pulse Batteries.

Notes: this isn't about limiting the turning rate of a turret, though it's compatible with such a feature. Also, the 'forward' point of any weapon is always forward, so this doesn't allow for broadsides and rear weapons. That'd be a great enhancement, but would require additional AI work. (Note that by 'forward' I mean the ship's facing + the convergence angle.)

[Elyssaen]

Reworking this from scratch given the new code changes for finite turret turn speed.