Fix gl2d snapshot rendering

[mikolalysenko]

Solves https://github.com/plotly/streambed/issues/6307

[etpinard]

Great. Looks like a solid first step. 🎉

Unfortunately, npm run test-image (after removing this line) is still struggling.

Running individual tests e.g. npm run test-image -- gl2d_stacked_subplots.json seems to behave fine. But, running multiple tests in a row leads to misplaced axes, hover spikes showing, and sometimes, even blank canvases.

@mikolalysenko Luckily, the problem is reproducible from within the test dashboard, by pasting the following in the console:

var list = [
  'gl2d_10',
  'gl2d_12',
  'gl2d_14',
  'gl2d_17',
  'gl2d_22',
  'gl2d_24',
  'gl2d_28',
  'gl2d_30',
  'gl2d_32',
];

list.forEach(function(mock) {
  Tabs.getMock(mock, function(err, fig) {
    var gd = Tabs.fresh(mock);  // appends new graph div with id set to mock name

    Plotly.plot(gd, fig.data, fig.layout).then(function() {
      Plotly.downloadImage(gd).then(function() {
        Plotly.purge(gd);  // this destroys the scene2d instances too!
        gd.parentNode.removeChild(gd);
      });
    });

  });
});

You can also use Plotly.toImage instead of Plotly.downloadImage to inspect the resulting dataURL.

Adding @monfera and @bpostlethwaite to the conversation who may have some insights on the problem.

[etpinard]

closed in favor of #735