Fix InexactErrors stemming from "rounding" in Ufixed numbers

Passing clim=(0,0.6) to ImageView worked for a Gray{Ufixed8} image,
but clim=(0.0.5) did not. That turned out to be because 0.6 is exactly
representable as a Ufixed8, but 0.5 is not.
This led to a scaling coefficient that produced numbers slightly
larger than 1, and could not be converted back to Ufixed8.