Gaps are ignored when in-between flat segments

[etpinard]

Consider

let regl = require('regl')({extensions: 'angle_instanced_arrays'})
let line2d = require('./')(regl)

const arr1 = [0, 1, 1, NaN, 2, 1, 3, 2]
const arr2 = [0, 3, 1, NaN, 2, 3, 3, 3]

const rng = [-1, -1, 4, 4]
const vp = [80, 80, 556, 350]

line2d.render({ thickness: 4, points: arr1, color: 'red', viewport: vp, range: rng, join: 'rect', overlay: true })
line2d.render({ thickness: 4, points: arr2, color: 'blue', viewport: vp, range: rng, join: 'rect', overlay: true })

which results in

where the gap in the blue trace at the 2nd (x,y) position is missing.

---

plotly.js example in -> plotly/plotly.js#3551