Make vision costs easier to use.

Previously, a unit's vision costs would fall back to movement costs only if no
vision costs were defined. Now this occurs "by terrain" -- the vision cost for a
specific terrain will fall back to the movement cost for that specific terrain,
even if the vision cost for another terrain had been defined. This makes it
easier to define, for example, being able to see over deep water without changing
vision in other cases.


git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56349 75b3775d-3801-0410-9e12-e2e43121521d