-use absolute x/y position and scale in MapPosition

- scale calculations look much nicer now, better always
   use 'double' unless you are sure about precision required
- finally got rid of zoomLevel relative coordinates
- cleanup MapPosition and MercatorProjection API functions

src/org/oscim/renderer/MapTile.java

@@ -24,6 +24,9 @@ import org.oscim.renderer.layer.TextItem;
  */
 public final class MapTile extends JobTile {
 
+	public double x;
+	public double y;
+
 	/**
 	 * Tile data set by TileGenerator.
 	 */
@@ -73,6 +76,8 @@ public final class MapTile extends JobTile {
 
 	MapTile(int tileX, int tileY, byte zoomLevel) {
 		super(tileX, tileY, zoomLevel);
+		this.x = (double)tileX / (1 << zoomLevel);
+		this.y = (double)tileY / (1 << zoomLevel);
 	}
 
 	/**