Only load image resources when needed.

- Stop loading all images on startup.
- Game images are preloaded before the song starts.
- Destroy skin images immediately after finishing a beatmap, potentially saving lots of memory.

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

src/itdelatrisu/opsu/states/Game.java

@@ -859,8 +859,10 @@ public class Game extends BasicGameState {
 		// set images
 		File parent = osu.getFile().getParentFile();
 		for (GameImage img : GameImage.values()) {
-			if (img.isGameImage() && img.setSkinImage(parent))
-				img.process();
+			if (img.isGameImage()) {
+				img.setDefaultImage();  // ensure that default image has been loaded
+				img.setSkinImage(parent);
+			}
 		}
 
 		// skip button

src/itdelatrisu/opsu/states/SongMenu.java

@@ -581,6 +581,9 @@ public class SongMenu extends BasicGameState {
 		// stop playing the theme song
 		if (MusicController.isThemePlaying() && focusNode != null)
 			MusicController.play(focusNode.osuFiles.get(focusNode.osuFileIndex), true);
+
+		// destroy skin images, if any
+		GameImage.destroySkinImages();
 	}
 
 	@Override