Fix: mask resize

[LutingWang]

mmcv takes (w, h) as input size, while mmdet.core.structures.BitmapMasks passes in (h, w). The mistake did not trigger pytest errors, since all the relevant tests let w == h.

A typical resized mask looks like

The bug is fixed and two more test cases are supplied in the pull request.

[codecov]

Codecov Report

Merging #4520 (b6d7aad) into master (3a9f5ef) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4520      +/-   ##
==========================================
- Coverage   62.09%   62.04%   -0.05%     
==========================================
  Files         241      241              
  Lines       18400    18400              
  Branches     3085     3085              
==========================================
- Hits        11426    11417       -9     
- Misses       6416     6426      +10     
+ Partials      558      557       -1     

Flag	Coverage Δ
unittests	62.01% <ø> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
mmdet/core/mask/structures.py	87.87% <ø> (ø)
mmdet/models/detectors/cornernet.py	94.87% <0.00%> (-5.13%)	⬇️
mmdet/models/dense_heads/corner_head.py	74.45% <0.00%> (-1.91%)	⬇️

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