Smaller refetch shifts: 1 tile at a time

With BUFFER_LONG = 1, refetch threshold was halfLong - 1 = 11,
which meant each refetch shifted the corridor centre by ~11 tiles.
From the camera's view that looked like the corridor suddenly
jumping 11 tiles forward — described as 'tiles jump to the right'.

BUFFER_LONG = 11 -> threshold = 1, so refetch fires as soon as
the camera crosses one tile beyond centre. Each refetch shifts by
~1 tile, which the user perceives as a smooth scroll rather than
a jump. Refetches now happen every 8 sec instead of every 92 sec,
but the load is small (only ~1 net new tile since browser caches
ESRI responses for 24h).

vendor/clouds3d.js

@@ -283,7 +283,7 @@ var GROUND_TRAVEL_COS = 0.0;      // cos(90 deg)
 var GROUND_TRAVEL_SIN = 1.0;      // sin(90 deg)
 var GROUND_CORRIDOR_LONG = 25;    // tiles along travel direction (~3.5 min of flight at uSpeed=1)
 var GROUND_CORRIDOR_PERP = 5;     // tiles perpendicular to travel
-var GROUND_BUFFER_LONG = 1;       // refetch when this close to forward edge
+var GROUND_BUFFER_LONG = 11;       // refetch when this close to forward edge
 var GROUND_BUFFER_PERP = 1;       // refetch when this close to side edge
 // Passau tile at zoom 11
 var GROUND_BASE_X = 1099;