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 194 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.

[HarelM]

@NathanMOlson any updates on this PR? Can we move it forward somehow?

[NathanMOlson]

This PR can be moved forward by fixing #4859. I don't know how to fix #4859.

Also, a better solution than the one in this PR would be to update the pixelmatch library to include the changes here: mapbox/pixelmatch#142. These changes have been approved by the maintainer, but have not been merged because they add processing time. I do not see a way to fix the pixelmatch library without adding processing time. In my opinion, MapLibre would be better served with a version of pixelmatch that detects transparency differences, even if it is slightly slower.

[HarelM]

In the past, a similar problem was solved by adding a border around the tiles to stitch different zoom level for tiles when terrain is on.
Is it possible that the border needs to be "bigger" to solve this issue (the tile is very far away).
I'm not sure it's the right fix, but wondering if it works solve the issue...