Preload game images when program initializes.

Reverted earlier change to load images immediately before a game begins, since there is a noticeable lag in some cases.

Other changes:
- Added 'preload' field to GameImage.
- Changed 'gameImage' field in GameImage to 'skinnable' (more suitable name).

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

src/itdelatrisu/opsu/Utils.java

@@ -188,8 +188,10 @@ public class Utils {
 
 		// initialize game images
 		GameImage.init(width, height);
-		GameImage.MENU_LOGO.setDefaultImage();
-		GameImage.MENU_BG.setDefaultImage();
+		for (GameImage img : GameImage.values()) {
+			if (img.isPreload())
+				img.setDefaultImage();
+		}
 
 		// initialize game mods
 		for (GameMod mod : GameMod.values())