Fix preview panel dragging experience

Preview panel dragging is clunky if zoom level above 1. The higher the zoom level, the worser it is.
This issue fixed by changing datatype that shifting the chunk position in preview panel from int to float. This happens probably rounding error and such

src/org/jmc/gui/PreviewPanel.java

@@ -63,7 +63,7 @@ public class PreviewPanel extends JPanel implements MouseMotionListener, MouseWh
 	/**
 	 * Offset of the map, as set by dragging the map around.
 	 */
-	private int shift_x,shift_y;
+	private float shift_x,shift_y;
 	/**
 	 * Zoom level of the map.
 	 */
@@ -449,8 +449,8 @@ public Rectangle getChunkBounds()
 	{
 		Rectangle ret=new Rectangle();
 
-		ret.x=(-shift_x/64)-1;
-		ret.y=(-shift_y/64)-1;
+		ret.x=((int)-shift_x/64)-1;
+		ret.y=((int)-shift_y/64)-1;
 		ret.width=(int) Math.ceil((getWidth()/zoom_level)/64.0)+1;
 		ret.height=(int) Math.ceil((getHeight()/zoom_level)/64.0)+1;
 
@@ -771,8 +771,8 @@ public void mouseDragged(MouseEvent e) {
 
 		if(moving_map)
 		{		
-			shift_x+=(x-last_x)/zoom_level;
-			shift_y+=(y-last_y)/zoom_level;
+			shift_x+=(float)(x-last_x)/zoom_level;
+			shift_y+=(float)(y-last_y)/zoom_level;
 
 			last_x=x;
 			last_y=y;