Fix computation of AutomationController steps

Fix computation of AutomationController steps

smallstep (resp. largestep) is intended to be the interface delta
corresponding to a desc.smallstep (resp. largestep) in internal scale,
and is computed by incrementing from desc.lower.

But ac->internal_to_interface(desc.lower) isn't necessarily zero. In
fact it currently is 0.5 / (M - m + 1) for integer parameters where M is
the maximum and m is the minimum possible value since it is the center
of the [0,1/(M-m+1)] interval.

Since the lower bound of the delta isn't always zero, don't ignore it
when computing the actual increment.