Feature/muted audio fixes #51

[mudar]

Hi Mattia,
This PR solves the problem I was facing when concatenating two dataSources where one would have audio+video tracks and the other would have a video track only (no audio track).
The new MutedAudioDataSource provides a silent sound track, which allows the concatenation to go on correctly.
How would you prefer to do the check that triggers the use of this new DataSource? Initially I thought it could be an AudioStrategy, or a builder option. For now (wip), I've added it in Builder.addDataSource(dataSource) by default, and it can be skipped by using Builder.addDataSource(track, dataSource).
I did many tests with the audio format values (bufferSize, periodSize, periodTime) and these values are the ones that worked for me 💯
thanks :)

[natario1]

Thanks @mudar this is great! :-) I'll take a look at the source soon. The idea of adding it automatically to the Builder seems very good to me. Let's do this under the hood

[natario1]

If you could just find a way to enable this only when needed that'd be great. Maybe at build() time?

I mean now if I pass a single file that is video only, I expect the output to be video only, so we shouldn't add muted audio in this case.

[mudar]

fixed, with the help of @cbernier2 :)
When build() is called, it calls buildAudioDataSources() to check if audio tracks are in a messy situation! The muted audio is added only when needed.

[natario1]

Just some small changes, thanks!

[natario1]

Thanks guys this is great. Great reference for a BlankVideo source as well, that seems quite easy now.

This source could be improved in readTrack(), because the last read will always give more data than it should (that is, durationUs is not an exact multiple of the period time). But we can address this later.