fix Overflow error in saving flows in io.save_masks

[pywugate]

fix overflow error (issue: #1256) by directly saving RGB uint8 flows[0] (discussion : #1257)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.89%. Comparing base (8483650) to head (4b4ffd7).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1261      +/-   ##
==========================================
- Coverage   48.96%   48.89%   -0.07%     
==========================================
  Files          16       16              
  Lines        3762     3767       +5     
==========================================
  Hits         1842     1842              
- Misses       1920     1925       +5     

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