Subtract the mean rotation of a vector field.
SF = subsbr2(F,DT,R0) is the same as subsbr, but also rotates the
resulting field of an angle THETA given by the temporal integration of
the angular velocity determined for each field. DT is the sampling time
(in sec.), and R0 = [X0, Y0] the center of rotation (middle of the
field if unspecified).