ForceGreeting always resets the dialogue window completely

Apparently if the talking actor is changed by ForceGreeting while you are still talking with another actor, it should update the window title and the topic list but keep the text spoken by the previous actor in the window around.

In the current implementation, the text will only not be reset if the speaking actor didn't change at all.

This vanilla quirk is planned to be used by TR designers to imitate three-way dialogue.