Give animations a chance (optional) to progress while fog/shroud clearing occurs.

This should help with bug #20324, maybe fix it.
The chance is optional because there are times when we do not want the delays, such
as when starting a scenario.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56169 75b3775d-3801-0410-9e12-e2e43121521d