Contourgl image tests

[dy]

I was exploring #1825 and realized that there is no image tests for contourgl type of plot, and simple setup is broken on my laptop:

<script src="https://cdn.plot.ly/plotly-latest.min.js"></script>
<script>
Plotly.plot(document.body, [{
  type: 'contourgl',
  z: [[1,2,3],[2,1,2]]
}]);
</script>

Codepen

Although, replacing plotly include with <script src="https://cdn.plot.ly/plotly-gl2d-latest.min.js"></script> fixes the plot.

Is contourgl excluded from main bundle by default or there is a bug? In particular, I cannot debug the issue mentioned in #1825

@etpinard

[dy]

Oh, that is excluded, I see. Ok.