Implemented "Flashlight" mod.

- Restricted view area using alpha maps and offscreen drawing. (credits: davedes)
- Added silver grades.

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

src/itdelatrisu/opsu/GameImage.java

@@ -327,7 +327,9 @@ public enum GameImage {
 		protected Image process_sub(Image img, int w, int h) {
 			return REPOSITORY.process_sub(img, w, h);
 		}
-	};
+	},
+	// TODO: ensure this image hasn't been modified (checksum?)
+	ALPHA_MAP ("alpha", "png", false, false);
 
 	/** Image file types. */
 	private static final byte