Inverts behavior of double touch drag gesture

Per Android guidelines, double touch drag up should decrease content scale and double touch drag down should increase content scale.

This is also the behavior in the official Google Maps Android application.

http://developer.android.com/design/patterns/gestures.html

vtm/src/org/oscim/layers/MapEventLayer.java

@@ -200,7 +200,7 @@ public class MapEventLayer extends Layer implements InputListener, GestureListen
 					return true;
 				}
 				// FIXME limit scale properly
-				mViewport.scaleMap(1 - my / (height / 6), 0, 0);
+				mViewport.scaleMap(1 + my / (height / 6), 0, 0);
 				mMap.updateMap(true);
 				mStartMove = -1;
 				return true;