fix createLinearGradient error when polygon ring is linear

[deyihu]

fix #2067

[codecov-commenter]

Codecov Report

Merging #2069 (5c2f29b) into master (9db0e54) will decrease coverage by 0.16%.
The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##           master    #2069      +/-   ##
==========================================
- Coverage   83.94%   83.78%   -0.16%     
==========================================
  Files         164      164              
  Lines       18085    18105      +20     
==========================================
- Hits        15181    15169      -12     
- Misses       2904     2936      +32     

Files Changed	Coverage Δ
...er/geometry/symbolizers/StrokeAndFillSymbolizer.js	70.53% <0.00%> (-15.34%)	⬇️

... and 4 files with indirect coverage changes

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