Fixes jMonkeyEngine#1249 - Don't hang up when destroy is called multiple times (jMonkeyEngine#1250)

Author: MeFisto94

jme3-lwjgl/src/main/java/com/jme3/system/lwjgl/LwjglAbstractDisplay.java

@@ -266,7 +266,11 @@ public void setAutoFlushFrames(boolean enabled){
         this.autoFlush = enabled;
     }
 
-    public void destroy(boolean waitFor){
+    public void destroy(boolean waitFor) {
+        if (needClose.get()) {
+            return; // Already destroyed
+        }
+
         needClose.set(true);
         if (waitFor)
             waitFor(false);