revealer: Use floor(), not ceil()

revealer: Use floor(), not ceil()

We use ceil() in measure(), so using it again will increase the
child's size whenever there is even a tiny rounding error.

This should also not make the size too small, because:
min = ceil(child_min * scale)
min / scale >= child_min
floor (min / scale) >= floor (child_min) = child_min
The last equality is because child_min is an integer.

Fixes #3137