prevCamera method

Implementation

Future<void> prevCamera() async {
  final cameras = await allCameras;
  if (cameras.isEmpty) {
    showToast("Keine Kameras verfügbar");
    return;
  }
  _currentCameraIndex =
      (_currentCameraIndex - 1 + cameras.length) % cameras.length;
  controller = await newController;
  notifyListeners();
}