Load beatmap background images in a new thread.

This eliminates the game-wide lag (100-200ms on my computer) when switching song nodes. Attempted to mask the loading time with a fade-in effect.

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>

src/itdelatrisu/opsu/ui/animations/AnimatedValue.java

@@ -77,6 +77,11 @@ public class AnimatedValue {
 		updateValue();
 	}
 
+	/**
+	 * Returns the total animation duration, in milliseconds.
+	 */
+	public int getDuration() { return duration; }
+
 	/**
 	 * Sets the animation duration.
 	 * @param duration the new duration, in milliseconds