- CSS background instead of pixi.js background

- Refactoring pixi.js code
- Resize game board on browser resize

public/css/game.css

@@ -5,4 +5,9 @@
     display: flex;
     justify-content: center;
     align-items: center;
+    background: url(/img/background.jpg) no-repeat center center fixed;
+    -webkit-background-size: cover; /* For WebKit*/
+    -moz-background-size: cover; /* Mozilla*/
+    -o-background-size: cover; /* Opera*/
+    background-size: cover; /* Generic*/
 }