Make the update of fog/shroud (after enabling delayed shroud updates) gradual.

This is possible as a side-effect of r56169. I think it looks kind of cool, but
if it turns out to be a bad idea, it can be easily reverted (just a matter of
changing a single "false" to "true").


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56170 75b3775d-3801-0410-9e12-e2e43121521d