Fix throwing an error when drawing rectangle annotations.

[manthey]

Fixes #692.

[codecov-io]

Codecov Report

Merging #693 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #693      +/-   ##
==========================================
+ Coverage   95.21%   95.24%   +0.03%     
==========================================
  Files          84       84              
  Lines        8919     8919              
==========================================
+ Hits         8492     8495       +3     
+ Misses        427      424       -3

Impacted Files	Coverage Δ
src/annotation.js	100% <ø> (ø)	⬆️
src/mapInteractor.js	96.11% <0%> (+0.28%)	⬆️
src/gl/vglRenderer.js	92.74% <0%> (+0.8%)	⬆️

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