Fix MetaDrive film size for straight SUMO maps

[lola831]

Description

This PR updates calculateFilmSize to clamp the map width/height to a minimum extent
before converting to pixels. When a SUMO convBoundary has zero extent in either
dimension (e.g., a straight road where width or height is 0), we would previously
compute a film size with a zero dimension, causing the 2D MetaDrive render to appear
blank. Clamping avoids this and keeps top-down rendering working for such maps.

Issue Link

Checklist

• I have tested the changes locally via pytest and/or other means
• I have added or updated relevant documentation
• I have autoformatted the code with black and isort
• I have added test cases (if applicable)

Additional Notes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.98%. Comparing base (e093379) to head (57838af).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #423      +/-   ##
==========================================
+ Coverage   89.86%   89.98%   +0.11%     
==========================================
  Files          54       54              
  Lines       13549    13556       +7     
==========================================
+ Hits        12176    12198      +22     
+ Misses       1373     1358      -15     

see 5 files with indirect coverage changes

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[Eric-Vin]

Changes look good, and confirmed to fix the issue we were experiencing. Thanks!