Caps at the beginning of the curve

[diyaayay]

Resolves #6816

Changes:
Sets let start=step; instead of let start=0; .
Added fix for both quadratic vertex segment and bezier vertex.

Screenshots of the change:
Before:

After:

sketch

function setup() {
  createCanvas(400, 400, WEBGL);
  background(255)
  noFill()
  stroke(0)
  strokeWeight(20)
  translate(-width/2, -height/2)
  beginShape()
  vertex(20, 20)
  quadraticVertex(
    280, 200,
    100, 380
  )
  endShape()
}

PR Checklist

• npm run lint passes
• Inline documentation is included / updated
• Unit tests are included / updated

[davepagurek]

Thanks for making these fixes!