Suggestion:Add a modify to audio set default

[i990049]

I've tested it many times and I'm still having problems. The video has three audio tracks, don't care about the picture quality and sound quality. the first audio track is MP3, the second audio track is AAC, the third audio track is OPUS, and the third audio track is default. I have chosen to set the second audio track of the output file AAC as the default audio track, but the default audio track of the output file is the first MP3, and the output file is bigger than the the original file. What is the problem? Here are the test file and debug.

https://wwi.lanzoup.com/ippIJ1jnnwdc