Cross Fade between music tracks within a playlist

[flmuse]

Before filing a feature request:

• Search existing open issues, specifically with the label ‘enhancement’:
  enhancement
• Search existing pull requests: https://github.com/google/ExoPlayer/pulls

When filing a feature request:

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[REQUIRED] Use case description

Whilst listening to a playlist using this plugin, moving to the next music track in a playlist sounds really jarring.

This is due to the silence and dip in volume between music tracks because most music files fade-out to silence as they reach their end and fade-in from silence. As a result, there is no smooth transition between music tracks.

Proposed solution

Crossfading between music tracks to make a mixed playlist sound much more seamless and natural-sounding transitions.

Would like to be able to set the duration of crossfades, such as 10 seconds.
In this case, a crossfade duration setting of 10 seconds would start to fade-out the current music track and fade in the next track in the playlist. This would result in a continuous mix and flow of music.

Crossfade function would be used when switching between music tracks such as when choosing the previous or next track, stop or start playing a music track.

I have working crossfade code in jquery syntax. If a developer knows both jquery and dart, they may utilize this to convert and implement the new functionailty into this plugin.

Alternatives considered

1. Transform a successfully working crossfade jquery script into Dart and integrate it into ExoPlayer
2. Edit all music tracks to remove the silences and fades at the beginning and end.

[tonihei]

This is a duplicate of #3428. Please see #3438 (comment) in particular for the most likely way to implement this.