Implement 'connectgaps' in scatter3d

[etpinard]

The connectgaps attribute determines whether gaps in the data (i.e. non-numeric values) are stitched together or not.

See http://codepen.io/etpinard/pen/mVNZEV

It would be nice to implement connectgaps starting in scatter3d traces and, down the road, all WebGL trace types.