Avoid cutting between keyframes

The way cutting currently happens creates files that don't have a keyframe at the start. Some players get confused by that. Also, this causes the title slide to remain visible when the audio start (until the first keyframe in the recording video).

Theoretically, this could be solved but re-encoding the part between the cut and the first keyframe. This is short (just a few seconds), so it should be fast. Determining exactly where to cut and re-encode would be more complicated.