resolve static method
Turns a URL/path/asset string into a cached source.
Implementation
static UMediaSource resolve(Object input) {
if (input is UMediaSource) return input;
final String value = input.toString();
final UMediaSource? cached = _cache[value];
if (cached != null) return cached;
final UMediaSource source = UAudio.source(value);
_cache[value] = source;
return source;
}