Fix misleading comment: image is X-flipped (NS-mirror), not 180°

The drawImage call uses width=-TILE, height=+TILE, which is an
X-only horizontal mirror. The old comment said '180 degrees'
which implied both axes flipped; the code is only horizontal.
NS axis = vertical axis, mirror across it = horizontal mirror.
Current state matches the user's observation: tile content
is NS-mirrored, X arrangement goes low->high left to right.

vendor/clouds3d.js

@@ -345,15 +345,12 @@ function drawTileAt(lng, perp, dx, dz, centerTileX, centerTileY) {
         return Promise.resolve(false);
     }
     return fetchTileImage(tileX, tileY).then(function(img) {
-        // Rotate image 180 degrees: draw with negative width and
-        // negative height, anchored at the tile's bottom-right corner
-        // (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.
-        // Mirror image across the NS axis (X-only flip). Image top
-        // (north) stays at canvas top, image bottom (south) stays
-        // at canvas bottom. Image left (west) -> canvas right, image
-        // right (east) -> canvas left.
+        // Mirror image across the NS axis (horizontal / X-flip):
+        // draw with negative width and positive height, anchored at
+        // the tile's right edge (canvasX + 256). Image top (north)
+        // stays at canvas top, image bottom (south) at canvas bottom.
+        // Image left (west) -> canvas right, image right (east) ->
+        // canvas left.
         groundCtx.drawImage(img, canvasX + GROUND_TILE_SIZE, canvasY,
             -GROUND_TILE_SIZE, GROUND_TILE_SIZE);
         groundLoadedTiles[tileX + '_' + tileY] = true;