preaim at blocked targets

[KyleAnthonyShepherd]

Currently, a HaveFreeLineOfFire check is performed during autotarget sweeps, and during weapon Update.
Targets that fail this check during the autotarget sweep are excluded, and targets that fail this check during Update are dropped as a valid target before aim scripts are called.

This causes behavior problems for units that "deploy" into a different configuration when they acquire a target. No matter if the deployment is as complicated as unfolding a missile tube, or as simple as raising a cannon barrel to correctly angle a shot.
If the HaveFreeLineOfFire check fails in the neutral, stowed away position, the unit AimWeaponX is never called and the unit never deploys their weapon into the configuration that may allow them to fire.
Previous examples in BAR include:
Sniper not raising rifle if there are small terrain obstructions.
beyond-all-reason/Beyond-All-Reason#1857
corvroc not raising their missile tube due to blocking friendly units.

Plasma bot in the back cannot fire over its friend because its guns are still flat, while the Plasma bot in the front can fire because it has previously angled up its guns

In addition, there are situations where you know the blocked target will be non-blocking soon, such as:
Rounding the corner around friendly walls,

A square of retreating units and the rear units could die soon, giving a free line of fire to the units in the middle

But no pre-aiming can occur due to the engine dropping blocked targets.

This PR adds a UnitDefWeapon boolean preaimAtBlockedTargets to allow those weapons to acquire and keep blocked targets. Actual weapon firing is still blocked by the relevant HaveFreeLineOfFire check, and non-blocked targets are only aimed at if there is no goodTargetUnit or badTargetUnit available, and if a goodTargetUnit or badTargetUnit becomes available, the next SlowUpdate Autotarget sweep will select that new target.

Video of bottom armrock with preaimAtBlockedTargets = true and corstorm without
https://github.com/beyond-all-reason/spring/assets/44480662/40bfce8d-8b0f-42c5-a12b-551540786e6a

[lhog]

Good change. Are any concerns to keep the PR in the draft state?

[TheSilverHornet]

+1 for needed functionality. I realise engine is in mothballs atm, but is there a possibilty to resurrect this PR?