Fix planar hull with several poins in the start/end sweep line

[blegat]

Closes #271

[codecov]

Codecov Report

Merging #281 (08a0bd5) into master (cfa465f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #281   +/-   ##
=======================================
  Coverage   88.34%   88.35%           
=======================================
  Files          37       37           
  Lines        2901     2910    +9     
=======================================
+ Hits         2563     2571    +8     
- Misses        338      339    +1     

Impacted Files	Coverage Δ
src/planar.jl	99.12% <100.00%> (-0.88%)	⬇️

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