Describe the bug
The GridTiler extractor with a slide with multiple tissue regions (arranged in a particular way, see example) uses overlapping extraction grids.

To Reproduce
Steps to reproduce the behavior:

from histolab.masks import TissueMask
from histolab.slide import Slide
from histolab.tiler import GridTiler
from histolab.data import breast_tissue

_, path = breast_tissue()
s = Slide(path, "pp")
mask = TissueMask()

extractor = GridTiler(
    tile_size=(512, 512),
    level=1,
    check_tissue=True,
    tissue_percent=80.0,
    pixel_overlap=0,
)

extractor.locate_tiles(s, mask)

Result:

Expected behavior
Every tissue region should have only one grid from which to extract from

Software (please complete the following information):

• OS: macOS 10.15.7
• Python Version 3.8.6
• histolab version: 0.2.5