Slow down flight + extend corridor for longer tile residency

Camera speed was 0.45 units/sec which on 13km/tile tiles = ~5850
km/h = supersonic. User reported flying through tiles too fast.

- drift speed 0.45 -> 0.12 units/sec (~1500 km/h, fast plane)
- corridor 15 -> 25 tiles along travel (~3.5 min between refetches
  at uSpeed=1, instead of ~12 sec)
- buffer 2 -> 3 tiles (refetch starts earlier so we always have
  tiles loaded before the camera reaches the edge)
- Updated JS-side drift calculation to match the new shader speed

Camera now leisurely follows the corridor; refetch only once every
couple of minutes.

vendor/clouds3d.js

@@ -136,7 +136,7 @@ var FRAG_SRC = [
         // Camera path: slow drift in X, cruising just above the cloud deck
         // like an airliner cockpit. No dives — we stay above so the
         // deck is always visible below us.
-        'float drift = uTime * uSpeed * 0.45;',
+        'float drift = uTime * uSpeed * 0.12;',
         // Fixed altitude well above the cloud ceiling (y=1.5).
         'float camY = 2.2;',
         'vec3 ro = vec3(0.0, camY, drift);',
@@ -275,9 +275,9 @@ var GROUND_TILE_SIZE = 256;
 // The corridor is aligned with both motion and look direction.
 var GROUND_TRAVEL_COS = 0.0;      // cos(90 deg)
 var GROUND_TRAVEL_SIN = 1.0;      // sin(90 deg)
-var GROUND_CORRIDOR_LONG = 15;    // tiles along the travel direction
+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 = 2;       // refetch when this close to forward edge
+var GROUND_BUFFER_LONG = 3;       // 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;
@@ -522,7 +522,7 @@ window.clouds3dRender = function(target) {
     gl.uniform2f(s.uGroundCenter, centerWorld.x, centerWorld.y);
     gl.uniform2f(s.uGroundSize, GROUND_CORRIDOR_LONG, GROUND_CORRIDOR_PERP);
 
-    var drift = now * speed * 0.45;
+    var drift = now * speed * 0.12;
     if (performance.now() - groundLastCheck > 1500) {
         groundLastCheck = performance.now();
         maybeRefetchGround(drift, drift * 0.7, gl);