Computes randomized quantile residuals for a fitted zero- or one-inflated
beta model. For the continuous observations the beta CDF is scaled by the
non-inflation probability; for the boundary observations the residual is
randomized uniformly over the probability atom, which is the standard
construction for a mixed discrete-continuous distribution. This is provided
because the generic residuals() method does not always handle the
inflation atom correctly, which can make every residual share the same sign.