exportImage method
Decodes srcPath with settings' size cap applied at decode time, crops
it to crop, re-encodes it, and returns the written file.
The caller owns the returned file.
Throws TransformException on any failure.
Implementation
@override
Future<File> exportImage(
String srcPath,
CropRect crop,
ImageEncodeSettings settings,
) async {
imageExports.add(
FakeImageExport(srcPath: srcPath, crop: crop, settings: settings),
);
await _settle();
return _write('img_${_counter++}.${settings.format.fileExtension}');
}