ffmpeg exports fail when LQ clips span the start or end of the master clip.

[kerryland]

eg:

A master track that is 30 minutes long
Clip 1 that begins 5 minutes before the master track, and is 10 minutes long
Clip 2 that begins 15 minutes into the master track, and is 8 minutes long
Clip 3 that begins 7 minutes before the end of the master track, and is 14 minutes long

      ------------------------------              master
 ----------                                       clip 1
                --------                          clip 2
                             --------------       clip 3

The resulting mp4 file for clip1 and clip 3 have messed up audio.

The fix is to get FFMpegTool.cs to export with arguments more like this:

string exportArgs = $"-y 
        -ss {queryStart} -i \"{videoFilePath}\"
        -ss {trackStart} -i \"{audioFilePath}\" 
        -c copy -map 0:v:0 -map 1:a:0 -t {duration} \"{targetFilePath}\"";