Re-add X-flip on image: NS-mirror tile content

The user wanted the tile content to be NS-mirrored (X-flip
on the image) so the corridor orientation matches. The labels
are drawn separately in normal orientation, so they read
normally. Now the arrangement has:
- canvasX = -perp + ... (X-swap): east tiles on screen right
- Image X-flipped (NS-mirror): image east at left of tile
- Labels in corners drawn normally: x=1102, y=707 readable

Combining these, the user looking at the screen sees:
- screen left: west tiles (low x), with image west on the right
- screen right: east tiles (high x), with image east on the left
- All labels readable

So the X coordinates read correctly when the eye scans left
to right across the corridor, and the image content is
NS-mirrored to match the corridor's direction of travel.

vendor/clouds3d.js

@@ -345,14 +345,13 @@ function drawTileAt(lng, perp, dx, dz, centerTileX, centerTileY) {
         return Promise.resolve(false);
     }
     return fetchTileImage(tileX, tileY).then(function(img) {
-        // Draw image with normal orientation (no X-flip). The image
-        // content within each tile is readable (place names, streets
-        // not mirrored). The arrangement uses the X-swap formula
-        // (canvasX = -perp + ...) to place east tiles on the right of
-        // the screen, so the X arrangement reads naturally left to
-        // right.
-        groundCtx.drawImage(img, canvasX, canvasY,
-            GROUND_TILE_SIZE, GROUND_TILE_SIZE);
+        // NS-mirror: X-flip the image so the tile content matches
+        // the corridor orientation (east tiles have their image east
+        // on the left of the drawn area, west tiles on the right).
+        // Labels are drawn separately in normal orientation, so the
+        // X-coordinates labels read normally.
+        groundCtx.drawImage(img, canvasX + GROUND_TILE_SIZE, canvasY,
+            -GROUND_TILE_SIZE, GROUND_TILE_SIZE);
         groundLoadedTiles[tileX + '_' + tileY] = true;
 
         // Thin border so the user can see where one tile ends and the