Let cluster projectiles inherit source_entity from parent

[Maxreader]

If trying to use a projectile with an action of type "cluster", whose action_delivery is of type "projectile", the child projectiles do not have any source_entity.

In the case of the mod I'm working on, the projectile target effect is scripted and relies on a player setting, which is found via the source_entity on the parent projectile.

This means I can't reliably find the player that launched the original projectile, so I can't reliably find the right setting to use.