Implemented volume-bg and master volume setting.

- All sounds are now multiplied by a master volume setting.
- Changed all default volume levels.
- Scrolling in the main menu and game states changes the master volume and displays a volume bar on the right side of the screen.
- "volume-bg.png" image by @kouyang.

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

src/itdelatrisu/opsu/GameImage.java

@@ -208,6 +208,12 @@ public enum GameImage {
 	SCORE_X ("score-x", "png"),
 
 	// Non-Game Components
+	VOLUME ("volume-bg", "png", false, false) {
+		@Override
+		protected Image process_sub(Image img, int w, int h) {
+			return img.getScaledCopy((h * 0.3f) / img.getHeight());
+		}
+	},
 	MENU_BACK ("menu-back", "png", false, false) {
 		@Override
 		protected Image process_sub(Image img, int w, int h) {