Implement connectGaps

[mikolalysenko]

This PR solves #281

For example, the following data:

{
  "data": [{
    "type": "scatter3d",
    "mode": "lines",
    "x": [0, 1, 2, 3, 4, 5],
    "y": [0, 1, 2, 3, 4, 5],
    "z": [2, 1, null, null, 0, 3],
    "connectgaps": false
  }],
  "layout": {
    "title": "connect gaps test",
  }
}

Would produce the following plot:

Gaps in line plots are implemented by inserting a pair of degenerate triangles into the triangle strip

[mikolalysenko]

Sorry for the messy commit history. I merged in the middle which screwed up the rebase.

[etpinard]

Ref commit in gl-line3d : gl-vis/gl-line3d@1e42375

[etpinard]

duplicate.

[etpinard]

and this line should be:

plots['scatter3d-connectgaps'] = require('@mocks/gl3d_scatter3d-connectgaps.json');

so that the corresponding image baseline is displayed properly.

[mikolalysenko]

whoops! that screwed up when I rebased

[etpinard]

@mikolalysenko Looking good. Thanks for doing this 🍻

We'll merge this in as soon as the minor edits ⏫ are done.

[etpinard]

nicely done.