Handle cases where more than 2 billion photons enter a pixel.

[schlafly]

When more than 2 billion photons are modeled as entering a pixel, the current code trips and dies. This caps the number of photons entering a pixel to 2**31 - 1.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.32%. Comparing base (244858c) to head (146c6d3).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #188      +/-   ##
==========================================
+ Coverage   90.02%   90.32%   +0.29%     
==========================================
  Files          17       17              
  Lines        2165     2170       +5     
==========================================
+ Hits         1949     1960      +11     
+ Misses        216      210       -6     

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