setPixel(vec, r, g, b) {
    this.context.fillStyle = "rgba(" + r + "," + g + "," + b + "," + this.opacity + ")";
    this.context.fillRect(vec.x, vec.y, 1, 1);
  }
Set the colour at a specific point on the canvas.

@param {Vector2} vec The pixel position on the canvas.
@param {Number} r The red colour-component.
@param {Number} g The green colour-component.
@param {Number} b The blue colour-component.
@param {Number} a The alpha colour-component.