Fix render test false negative

[NathanMOlson]

Fixes #4862. Merging this change will cause the terrain/fog render test to fail (as it should), due to #4859.

The pixelmatch library does not detect a difference between transparent and white. To work around this, the horizon and fog colors have been changed to not be white.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.95%. Comparing base (86ed382) to head (b869ce9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4864      +/-   ##
==========================================
- Coverage   87.66%   86.95%   -0.71%     
==========================================
  Files         265      265              
  Lines       37937    37937              
  Branches     2407     2485      +78     
==========================================
- Hits        33257    32988     -269     
- Misses       3608     3828     +220     
- Partials     1072     1121      +49     

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[HarelM]

I think we should not merge a failing render test though, we should update the image or fix the root cause of this test as oart of this PR.