fix triangle() orientation in webgl mode

[inaridarkfox4231]

Since the direction of triangle() is the front direction regardless of the order of specifying vertices, we make sure that the order of specifying vertices is properly reflected.

Resolves #6262

Changes:

When calculating the matrix, by inserting the sign of the direction into the (3,3) component, to make sure that the order of specifying the vertices will be reflected.

Screenshots of the change:

triangle()_debug

function setup() {
  createCanvas(400, 400, WEBGL);

  noStroke();
  pointLight(color("lime"), 0, 0, 50);

  // clockwise orientation. normal: (0, 0, 1)
  quad(-200, -200, 0, -200, 0, 0, -200, 0);
  // counter-clockwise orientation. normal: (0, 0, -1)
  quad(200, -200, 0, -200, 0, 0, 200, 0);

  // clockwise orientation. normal: (0, 0, 1)
  triangle(0, 0, 0, 200, -200, 200);
  // counter-clockwise orientation. normal: (0, 0, 1), the same.
  triangle(0, 0, 0, 200, 200, 200);
}

before

after

PR Checklist

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

[inaridarkfox4231]

Thanks for merge!