When computing required/invalidated regions, calculate the bounding
box based on pixel corners, rather than pixel centers, to allow for
some error in the sampled input coordinates.  Otherwise, in some
cases, we sample pixels outside the required input region during
process() (and, conversely, we probably also miss affected pixels
when determining the invalidated region.)

Update the reference output of the "clones" test composition, which
is affected by this change.