Better handling of missiles splitting to other missiles (#8603)

- Pass a shooter separate to owner into `BulletType#create`. `killShooter` kills the owner while the shooter is passed into the missile ai
- `killShooter` no longer kills already dead units
- `MissileAI` no longer aims at shooter's aim pos if the shooter is dead

core/src/mindustry/entities/bullet/BulletType.java

@@ -554,7 +554,7 @@ public class BulletType extends Content implements Cloneable{
 
     public void init(Bullet b){
 
-        if(killShooter && b.owner() instanceof Healthc h){
+        if(killShooter && b.owner() instanceof Healthc h && !h.dead()){
             h.kill();
         }
 
@@ -726,6 +726,10 @@ public class BulletType extends Content implements Cloneable{
     }
 
     public @Nullable Bullet create(@Nullable Entityc owner, Team team, float x, float y, float angle, float damage, float velocityScl, float lifetimeScl, Object data, @Nullable Mover mover, float aimX, float aimY){
+        return create(owner, owner, team, x, y, angle, damage, velocityScl, lifetimeScl, data, mover, aimX, aimY);
+    }
+
+    public @Nullable Bullet create(@Nullable Entityc owner, @Nullable Entityc shooter, Team team, float x, float y, float angle, float damage, float velocityScl, float lifetimeScl, Object data, @Nullable Mover mover, float aimX, float aimY){
         if(spawnUnit != null){
             //don't spawn units clientside!
             if(!net.client()){
@@ -738,17 +742,19 @@ public class BulletType extends Content implements Cloneable{
                 }
                 //assign unit owner
                 if(spawned.controller() instanceof MissileAI ai){
-                    if(owner instanceof Unit unit){
+                    if(shooter instanceof Unit unit){
                         ai.shooter = unit;
                     }
 
-                    if(owner instanceof ControlBlock control){
+                    if(shooter instanceof ControlBlock control){
                         ai.shooter = control.unit();
                     }
 
                 }
                 spawned.add();
             }
+            //Since bullet init is never called, handle killing shooter here
+            if(killShooter && owner instanceof Healthc h && !h.dead()) h.kill();
 
             //no bullet returned
             return null;