Debug labels on each tile: N/S markers + x/y coords

For verifying the corridor arrangement, draw labels on every
loaded tile (per drawTileAt):
- 'N' at the canvas top edge of the tile (= image north after UNPACK_FLIP_Y_WEBGL)
- 'S' at the canvas bottom edge (= image south)
- 'x=NNN y=NNN' at the centre with the actual ESRI tile coords

This lets you see at a glance which canvas direction maps to which
world direction. Combined with #flight=N|... and #fetch=N|... you
can describe exactly what is wrong (e.g. N pointing in screen-up
direction when it should point at the corridor front).

vendor/clouds3d.js

@@ -336,6 +336,29 @@ function drawTileAt(lng, perp, dx, dz, centerTileX, centerTileY) {
     return fetchTileImage(tileX, tileY).then(function(img) {
         groundCtx.drawImage(img, canvasX, canvasY);
         groundLoadedTiles[tileX + '_' + tileY] = true;
+
+        // Debug labels: 'N' at the image's north edge (top of canvas
+        // tile area after UNPACK_FLIP_Y_WEBGL), 'S' at south edge.
+        // Tile coords X,Y at the centre so we can verify the canvas
+        // arrangement matches the world directions.
+        groundCtx.fillStyle = 'rgba(255,255,255,0.85)';
+        groundCtx.strokeStyle = 'rgba(0,0,0,0.85)';
+        groundCtx.lineWidth = 3;
+        groundCtx.font = 'bold 48px sans-serif';
+        groundCtx.textAlign = 'center';
+        // N at canvas top of tile area (= north in image)
+        groundCtx.strokeText('N', canvasX + 128, canvasY + 56);
+        groundCtx.fillText('N', canvasX + 128, canvasY + 56);
+        // S at canvas bottom of tile area
+        groundCtx.strokeText('S', canvasX + 128, canvasY + 240);
+        groundCtx.fillText('S', canvasX + 128, canvasY + 240);
+        // Tile coords
+        groundCtx.font = 'bold 20px monospace';
+        groundCtx.strokeText('x=' + tileX + ' y=' + tileY,
+            canvasX + 128, canvasY + 140);
+        groundCtx.fillText('x=' + tileX + ' y=' + tileY,
+            canvasX + 128, canvasY + 140);
+
         return true;
     }).catch(function() {
         // On failure draw a neutral gray so the gap is at least