Properly mask rows and columns when fetching balanced pixels as dense matrices

[robomics]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 84.90566% with 8 lines in your changes missing coverage. Please review.

Project coverage is 74.63%. Comparing base (784477d) to head (b42d6c7).

Files with missing lines	Patch %	Lines
...e/hictk/transformers/impl/to_dense_matrix_impl.hpp	81.81%	3 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #248      +/-   ##
==========================================
+ Coverage   74.58%   74.63%   +0.05%     
==========================================
  Files         144      144              
  Lines       12059    12117      +58     
  Branches     1385     1399      +14     
==========================================
+ Hits         8994     9044      +50     
- Misses       1946     1949       +3     
- Partials     1119     1124       +5     

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[robomics]

Closing in favor of #251.