allow looped music to fade back into itself at LoopEnd

[pzhang87]

In the current way to implement music, we need to provide both a LoopStart and a LoopEnd point (in samples); after reaching LoopEnd, the music will immediately jump back to LoopStart. This works fine in the case of some tracks, but for audio that features initial pickup notes or long reverb trails at the end it would sound smoother if these sections were able to be crossfaded into each other instead of a jump cut.

[james7132]

Loops need to be reimplemented in the BGM system.

https://feedback.unity3d.com/suggestions/loop-audio-from-point

Looks like the Unity end support will not be coming anytime soon.