Returns the perp dot product of this and the other vector.
Selected identities that the perpDot satisfies:
a perpDot b = -b perpDot a
a perpDot b = a.magnitude * b.magnitude * sin(theta)
(a perpDot b)^2 + (a dot b)^2 = a.magnitude^2 * b.magnitude^2
where theta is the angle from vector a to vector b.