gpuOf method
path's GPU material, with what of its maps has arrived and
overrides (an entity's values, by MaterialFile.animatableKeys)
on top; null until the file is read, or when it is broken.
Implementation
GpuMaterial? gpuOf(String path, {Map<String, Object?> overrides = const {}}) {
final made = _madeOf(path);
if (made == null) return null;
final shared = made.gpu ??= _make(made, made.file);
if (overrides.isEmpty) return shared;
final key = (path, _keyOf(overrides));
final known = _overridden[key];
if (known != null) return known;
if (_overridden.length >= maxOverridden) _overridden.clear();
return _overridden[key] = _make(made, made.file.withValues(overrides));
}