Walls of lava

Add "walls of lava", basically lava which blocks vision and
require a bit more than just levitation or flight to move through.

No levels use this yet, as testing isn't thorough enough.

src/vision.c

@@ -161,6 +161,7 @@ does_block(int x, int y, struct rm *lev)
     if (gs.seethru != 1) {
 #endif
     if (lev->typ == CLOUD || IS_WATERWALL(lev->typ)
+        || lev->typ == LAVAWALL
         || (Underwater && is_moat(x, y)))
         return 1;
 #ifdef DEBUG