Describe the bug
GridTiler() finds glass artifacts as the biggest tissue box.

To Reproduce
Steps to reproduce the behavior:

1. from histolab.slide import Slide
   from histolab.tiler import GridTiler
2. grid_tiles_extractor = GridTiler(
   tile_size=(512, 512),
   level=3,
   check_tissue=False,
   tissue_percent=20.0,
   prefix=prefix + '_',
   suffix=".png"
   )
3. grid_tiles_extractor.locate_tiles(
   slide=img,
   scale_factor=24, # default
   alpha=128, # default
   outline="red", # default
   )
4. 

Software (please complete the following information):

• OS: Ubuntu
• Python Version: 3.8
• histolab version: 0.5.1