fix: box computing when assume straight pages is false

[charlesmindee]

This PR fixes the following:

• angle computation in visualization.py
• angle computation in fit_rbbox
• Polygon expansion for rbbox: before this PR, we computed the area & perimeter of the polygon for the Vatti clipping algorithm from the raw polygon, now we compute it from the rotated rectangle which is more accurate and allow us to match the boxes when we have assume_straight_pages = False and assume_straight_pages = True.

before:

now:

[fg-mindee]

Thanks a lot! Only a small question about the box fitting function in comment

[codecov]

Codecov Report

Merging #720 (44c3686) into main (cf97d9e) will decrease coverage by 0.03%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##             main     #720      +/-   ##
==========================================
- Coverage   96.12%   96.09%   -0.04%     
==========================================
  Files         124      125       +1     
  Lines        4648     4714      +66     
==========================================
+ Hits         4468     4530      +62     
- Misses        180      184       +4     

Flag	Coverage Δ
unittests	96.09% <91.66%> (-0.04%)	⬇️

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Impacted Files	Coverage Δ
doctr/utils/visualization.py	92.03% <0.00%> (ø)
...dels/detection/differentiable_binarization/base.py	90.37% <100.00%> (-0.19%)	⬇️
doctr/utils/geometry.py	98.00% <100.00%> (ø)
doctr/models/builder.py	96.49% <0.00%> (-2.64%)	⬇️
doctr/io/pdf.py	100.00% <0.00%> (ø)
doctr/datasets/__init__.py	100.00% <0.00%> (ø)
doctr/datasets/svhn.py	94.73% <0.00%> (ø)
doctr/models/detection/linknet/base.py	87.12% <0.00%> (+1.98%)	⬆️

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[fg-mindee]

Let's keep in mind that the angle is off by 90°, but the rest is fine!