function dot(a, b) {
    return (a[0] * b[0]) + (a[1] * b[1]) + (a[2] * b[2]);
  }
Computes the dot product of two vectors; assumes both vectors have
three entries.
@param {module:twgl/v3.Vec3} a Operand vector.
@param {module:twgl/v3.Vec3} b Operand vector.
@return {number} dot product
@memberOf module:twgl/v3