Add a screenshot button in the examples header.

[manthey]

I don't know if we want this added to the examples, but it helps show off the feature, and is simpler than doing a proper screenshot widget. If you think it is undesirable, we can get rid of the PR.

Another way to do this is to just show a save dialog with the screenshot (and not show a thumbnail of it). In this version you can right-click on the image and copy it instead of being forced to download it.

[codecov-io]

Codecov Report

Merging #669 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #669      +/-   ##
==========================================
+ Coverage   90.63%   90.64%   +0.01%     
==========================================
  Files          84       84              
  Lines        8725     8736      +11     
==========================================
+ Hits         7908     7919      +11     
  Misses        817      817

Impacted Files	Coverage Δ
src/map.js	98.78% <100%> (+0.01%)	✅
src/util/init.js	84.66% <100%> (ø)	✅

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[jbeezley]

I think the feature is okay. Just a couple minor notes:

1. The button shouldn't appear in the header on the main examples page.
2. It should use a different pointer when mousing over the element.