fix initial option to make the AC3 track the first audio track

[sbholmes]

The initial option doesn't work properly. From my reading, it should make the AC3 track the first audio track in the file.

For example your test mkv file has track 1 as the video and track 2 as the DTS audio, the --initial option should put the new AC3 track as track 2, and shunt the DTS track to 3. The video should stay as track 1. This doesn't happen.

Currently if you use --initial on your test mkv file you end up with the DTS track as track 1, the AC3 track as track 2 and the video track as track 3! This confuses some players like my TV.

This fix makes the AC3 track the first audio track in the file when using the --initial option.