I am using Camtasia for Mac. I am a rookie . I put a clip from an old movie on the timeline and clipped out a certain character. I plan to replace it with green screen stuff. however, from the beginning, I note when I play over the clips the initial second is green! also if I scroll backwards on any clip, it is green. why is that? this is nothing about green screen additions. I noted this from the beginning on the timeline. what can be done?