Minor change to GDL

[Stan Shebs]

I just checked in a number of fixes to vision code, mostly inspired
by weirdnesses in africa-1850.  One of those involved occupant-vision,
which governs whether occupants can see anything or not.  This was
previously a boolean, but in the new vision code, it was trivial to
generalize it a bit, so I made it a modifier to the changes of seeing
something; so now its default is 100, while 0 causes occupant blindness,
and other values either improve or degrade an occupant's vision.

This doesn't affect any existing library games, since only galaxy2
uses this, and it sets the value to 0 for everybody (spacecraft in
hangar bays not having their sensors deployed!)

Stan