New interface global

[Hans Ronne]

I have added a new gvar, "terrain-transitions". By setting it to false, a
game designer can turn off terrain transitions in a game where they look
silly. It works exactly like the "meridians" and "feature-boundaries" gvars
which are used to turn off (or on) those two interface features.

These gvars will override any preferences settings that the user has for
the corresponding interface features, but he can still turn them on (or
off) by hand. If he wants a silly-looking interface, that's his privilege.

Hans