Fix flickering on lines

[ibesora]

This PR fixes line flickering as seen in #5025
The problem was that we were calling a function that changed the stencil value used even if we were not using the result of it. Wrapping everything with an if block made it work

Launch Checklist

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality.
• Document any changes to public APIs.
• Post benchmark scores.
• Add an entry to CHANGELOG.md under the ## main section.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.11%. Comparing base (94e2729) to head (d00152c).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5094      +/-   ##
==========================================
+ Coverage   89.06%   89.11%   +0.05%     
==========================================
  Files         269      269              
  Lines       38386    38392       +6     
  Branches     2370     2382      +12     
==========================================
+ Hits        34189    34214      +25     
+ Misses       3188     3173      -15     
+ Partials     1009     1005       -4     

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[HarelM]

I have little faith someone will read this comment, and that it will reflect what the method is doing in the future.

I would consider splitting it to getting current and advancing the next or rename it to reflect that it changes the state.

[HarelM]

I think it will be better to inline the relevant geojson line to prevent this tests from failing in the future if this file is not available.
Also we probably need only one line, so it should be straight forward...
It's also worth making sure the test fails without the fix.