function dot$4(a, b) {
    return a[0] * b[0] + a[1] * b[1] + a[2] * b[2];
  }
Calculates the dot product of two vec3's

@param {ReadonlyVec3} a the first operand
@param {ReadonlyVec3} b the second operand
@returns {Number} dot product of a and b