function dot(a, b) {
  return a[0] * b[0] + a[1] * b[1] + a[2] * b[2] + a[3] * b[3];
}
Calculates the dot product of two vec4's

@param {vec4} a the first operand
@param {vec4} b the second operand
@returns {Number} dot product of a and b