Add O/W mirror: 180° rotation (image content only)

User wants the O/W axis mirrored within each tile. With
the previous Y-only flip, East was still on the right and West
on the left. Adding an X-flip makes it 180° rotation:
- Image North -> canvas bottom, South -> canvas top
- Image East -> canvas left, West -> canvas right

Labels N/S were already positioned at canvas bottom/top to match
the Y-flipped image, so they stay where they are. Tile arrangement
in the corridor is unchanged (still controlled by canvasX/canvasY
formulas with no swap on either axis).

vendor/clouds3d.js

@@ -346,11 +346,12 @@ function drawTileAt(lng, perp, dx, dz, centerTileX, centerTileY) {
         // (canvasX + 256, canvasY + 256). After this, image north
         // (high lat) is at canvas bottom and image south at canvas
         // top; image east at canvas right, image west at canvas left.
-        // Flip image only on the Y axis (NS mirror). Image top
-        // (north) -> canvas bottom, image bottom (south) -> canvas
-        // top. X axis (EW) is preserved.
-        groundCtx.drawImage(img, canvasX, canvasY + GROUND_TILE_SIZE,
-            GROUND_TILE_SIZE, -GROUND_TILE_SIZE);
+        // Flip image 180° (both X and Y). Image top (north) -> canvas
+        // bottom, image bottom (south) -> canvas top, image left
+        // (west) -> canvas right, image right (east) -> canvas left.
+        // Labels N/S already match the Y-flipped orientation.
+        groundCtx.drawImage(img, canvasX + GROUND_TILE_SIZE, canvasY + GROUND_TILE_SIZE,
+            -GROUND_TILE_SIZE, -GROUND_TILE_SIZE);
         groundLoadedTiles[tileX + '_' + tileY] = true;
 
         // Debug labels: 'N' at the image's north edge (top of canvas