Move mock VGL functions to util

[manthey]

This will make in convenient for other projects to test geojs in non-webgl browsers.

Also, this now allow multiple contexts to be mocked.

[manthey]

I expect the travis tests to fail since Firefox 53 has been released. It has changed something in the button padding that is messing things up.

[jbeezley]

I'm okay with the changes. Do you need to update the screenshots for Firefox 53?

[codecov-io]

Codecov Report

Merging #688 into master will decrease coverage by <.01%.
The diff coverage is 91.3%.

@@            Coverage Diff             @@
##           master     #688      +/-   ##
==========================================
- Coverage   95.24%   95.23%   -0.01%     
==========================================
  Files          83       84       +1     
  Lines        8850     8919      +69     
==========================================
+ Hits         8429     8494      +65     
- Misses        421      425       +4

Impacted Files	Coverage Δ
src/util/index.js	100% <100%> (ø)	⬆️
src/util/mockVGL.js	91.17% <91.17%> (ø)
src/polyfills.js	25% <0%> (+10%)	⬆️

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