text: Title: [Feature Request] Add multichannel support to RandomShadow
  Body: RIght now we have:
  
  ```python
  mask = np.zeros_like(img, dtype=np.uint8)
  cv2.fillPoly(mask, vertices_list, (max_value, max_value, max_value))
  ```
  in `add_shadow` function.
  
  `cv2.fillPolly`, does not work with the number of channels > 4
  
  => 
  
  1. if number of channels is larger than 4, create mask for 1 channel and replicate it to the desired number of channels.
  2. check that everything works for grayscale image
  3. tests
  4. update docstrings in `add_shadow` and `RandomShadow`
  