adoptSoLoud function
A backend over soloud, a SoLoud engine the game already holds,
reporting through onIssue as SoLoudBackend.new does.
In a library of its own, package:flutter3d_audio/soloud.dart,
because it names flutter_soloud's SoLoud: a major of that package
changes this signature, so the library follows flutter_soloud's major
(SUPPORT.md) while the rest of the package keeps the engine's promise.
Implementation
SoLoudBackend adoptSoLoud(SoLoud soloud, {IssueSink? onIssue}) =>
SoLoudBackend._(soloud, onIssue);