- draw up to two parent proxies for current tile (not just one)

- nicer shader for devices not having GL_OES_standard_derivatives
- fix: check for min/max zoom position

src/org/mapsforge/database/mapfile/MapDatabase.java

@@ -302,7 +302,8 @@ public class MapDatabase implements IMapDatabase {
 	public FileOpenResult openFile(File mapFile) {
 		try {
 			if (mapFile == null) {
-				throw new IllegalArgumentException("mapFile must not be null");
+				// throw new IllegalArgumentException("mapFile must not be null");
+				return new FileOpenResult("no file!");
 			}
 
 			// make sure to close any previously opened file first