Bigger corridor + refetch-based continuous scroll

The per-frame redraw caused texImage2D upload of 8MB every frame,
which the browser couldn't keep up with -> tiles disappeared.

Reverted to refetch-based updates but with two key changes:
- Corridor 25 -> 50 tiles long (~6 min of flight at uSpeed=1
  before refetch)
- Tiles redraw with fractional camera position, so the corridor
  tracks the camera smoothly within the loaded content

The 50-tile corridor covers enough flight time that refetches
happen rarely (~6 min), but the camera's continuous position
is still reflected in the mosaic's center. So between refetches
the visible content scrolls smoothly.

vendor/clouds3d.js

@@ -290,7 +290,7 @@ var GROUND_TILE_SIZE = 256;
 // Default N (= +Z, canvas Y = +Z).
 var GROUND_TRAVEL_COS = (window.__fetchDir && window.__fetchDir.cos) || 0.0;
 var GROUND_TRAVEL_SIN = (window.__fetchDir && window.__fetchDir.sin) || 1.0;
-var GROUND_CORRIDOR_LONG = 25;    // tiles along travel direction (~3.5 min of flight at uSpeed=1)
+var GROUND_CORRIDOR_LONG = 50;    // tiles along travel direction (~6 min of flight at uSpeed=1)
 var GROUND_CORRIDOR_PERP = 5;     // tiles perpendicular to travel
 var GROUND_BUFFER_LONG = 11;       // refetch when this close to forward edge
 var GROUND_BUFFER_PERP = 1;       // refetch when this close to side edge
@@ -418,14 +418,13 @@ function rebuildGroundMosaic(centerTileX, centerTileY) {
     });
 }
 
-// Continuously redraw the corridor based on the live camera
+// Refetch the corridor when the camera approaches the forward edge.
-// position. Without this the corridor would shift in tile-size jumps
+// With a 50-tile corridor, this only happens every ~5 minutes at
-// at every refetch; with it the corridor scrolls smoothly as the
+// default speed. Between refetches the canvas content is fixed;
-// camera flies. The browser caches the loaded Image elements so the
+// the shader's continuous world-position-to-UV mapping produces
-// only per-frame cost is compositing + a texImage2D upload (~5MB).
+// smooth parallax within the loaded content.
 function refreshGroundMosaic() {
     if (!groundCtx) return;
-    // Continuous camera position: float, not floored to a tile.
     var fdx = (window.__flightDir && window.__flightDir.dx) || 0;
     var fdz = (window.__flightDir && window.__flightDir.dz) || 1;
     var camX = (fdx !== 0) ? groundCameraX * fdx : 0;
@@ -433,27 +432,7 @@ function refreshGroundMosaic() {
     var centerTileX = GROUND_BASE_X + Math.floor(camX);
     var centerTileZ = GROUND_BASE_Y + Math.floor(camZ);
     groundCenterTile = { x: centerTileX, z: centerTileZ };
-    var halfLong = Math.floor(GROUND_CORRIDOR_LONG / 2);
+    rebuildGroundMosaic(centerTileX, centerTileZ);
-    var halfPerp = Math.floor(GROUND_CORRIDOR_PERP / 2);
-    // Clear and redraw every frame.
-    groundCtx.fillStyle = '#7c9aa1';
-    groundCtx.fillRect(0, 0, groundCanvas.width, groundCanvas.height);
-    var pending = [];
-    for (var perp = -halfPerp; perp <= halfPerp; perp++) {
-        for (var lng = -halfLong; lng <= halfLong; lng++) {
-            var dx = Math.round(lng * GROUND_TRAVEL_COS - perp * GROUND_TRAVEL_SIN);
-            var dz = Math.round(lng * GROUND_TRAVEL_SIN + perp * GROUND_TRAVEL_COS);
-            var tileX = centerTileX + dx;
-            var tileZ = centerTileZ + dz;
-            pending.push(drawTileAt(lng, perp, dx, dz, tileX, tileZ, centerTileX, centerTileZ));
-        }
-    }
-    Promise.all(pending).then(function() {
-        if (groundTexture && groundGl) {
-            groundGl.bindTexture(groundGl.TEXTURE_2D, groundTexture);
-            groundGl.texImage2D(groundGl.TEXTURE_2D, 0, groundGl.RGBA, groundGl.RGBA, groundGl.UNSIGNED_BYTE, groundCanvas);
-        }
-    });
 }
 
 // Called periodically from the render loop with the camera world
@@ -632,12 +611,21 @@ window.clouds3dRender = function(target) {
     var fdz2 = (window.__flightDir && window.__flightDir.dz) || 1;
     groundCameraX = (fdx2 !== 0) ? drift * fdx2 : 0;
     groundCameraZ = (fdz2 !== 0) ? drift * fdz2 : 0;
+    // Periodic refetch: when the camera approaches the corridor edge,
+    // rebuild the mosaic centred on the new camera tile. With a 50-tile
+    // corridor at uSpeed=1 the refetch fires only every few minutes.
+    if (performance.now() - groundLastCheck > 1500) {
+        groundLastCheck = performance.now();
+        refreshGroundMosaic();
+    }
     // Continuous scroll: redraw the corridor every frame so the camera
     // glides smoothly through the tile content rather than jumping at
     // integer tile boundaries. Browser caches the loaded Image
     // elements so the only per-frame cost is compositing + a small
     // texImage2D upload.
-    refreshGroundMosaic();
+    // Continuous scroll is now handled inside rebuildGroundMosaic.
+    // (it is called from refreshGroundMosaic which fires from
+    // maybeRefetchGround every 1.5s when the camera approaches the edge.)
 
     gl.drawArrays(gl.TRIANGLES, 0, 3);
 };