add per-tile min/max elevation support

Assets/Scripts/GeoDataUtils/GeoTileMetadata.cs

@@ -12,15 +12,18 @@ public class GeoTileMetadata : MonoBehaviour
     public double ymin;
     public double globalMin;
     public double globalMax;
+    public double tileMin;
+    public double tileMax;
 
     /// <summary>
     /// Returns the world position this tile should be placed at, given a global origin.
     /// </summary>
     public Vector3 GetWorldPosition(double originX, double originY)
     {
+        double baseY = tileMax > tileMin ? tileMin : globalMin;
         return new Vector3(
             (float)(xmin - originX),
-            (float)globalMin,
+            (float)baseY,
             (float)(ymin - originY)
         );
     }