Add random stage

public/src/css/songbg.css

@@ -1,27 +1,32 @@
-#songbg{
+#songbg,
+#songbg>*,
+#song-stage{
 	position: absolute;
 	right: 0;
 	bottom: 0;
 	left: 0;
 	width: 100%;
+}
+#songbg{
 	max-width: calc(100vh / 9 * 32);
 	height: 50.1%;
 	background-color: #000;
+}
+#songbg>*{
+	top: 0;
+}
+#songbg,
+#songbg>*{
 	background-size: cover;
 	background-position: center;
 }
+#song-stage{
+	height: calc(44 / 720 * 100vh);
+	background-position: center;
+}
 .portrait #songbg{
 	height: 63.4%;
 }
-#songbg>*{
-	position: absolute;
-	top: 0;
-	right: 0;
-	bottom: 0;
-	left: 0;
-	background-size: cover;
-	background-position: center;
-}
 .game-paused *{
 	animation-play-state: paused !important;
 }