Duke3D: Introduce actor event 'EVENT_PREACTORDAMAGE'

This event is executed when an actor runs A_IncurDamage(), and is placed just before subtracting htextra from extra and updating the actor's owner (i.e. just before damage is applied to the actor).

For useractors, this normally occurs when ifhitweapon is called.

Set RETURN to a nonzero value to skip applying the damage. This will also cause the ifhitweapon conditional to skip the branch. Note that doing so will not automatically reset htextra to 0.

This event has some significant differences to EVENT_DAMAGESPRITE that make it worthwhile having:

• This event is only executed when an actor actually applies damage, rather than whenever a sprite is hit by a projectile. Allows you to cancel applying damage and taking the ifhitweapon branch.
• This event does not suffer from the problem of radius damage potentially reducing htextra before damage is applied. This makes it possible to accurately detect whether an actor will die from the damage taken, which is not possible with EVENT_DAMAGESPRITE.
• No need to declare the picnum being hit using damageeventtile, runs for every useractor that calls ifhitweapon.
• Current player is defined within the event. This is not defined with the DAMAGESPRITE events.