preResolveAll abstract method
Resolves every source in sources (download, read, decode) so that
resolvedFor and decodedImageFor can answer synchronously during the
frame loop.
Runs before the first frame and is idempotent: resolving the same source
twice does the work once. Throws a FluvieRenderException when a source
cannot be resolved.
Implementation
Future<void> preResolveAll(Iterable<MediaSource> sources);