Flush GL pipeline for pbuffer after frame (required because there's no backbuffer)

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

@@ -127,6 +127,10 @@ public class LwjglOffscreenBuffer extends LwjglContext implements Runnable {
 
         renderer.postFrame();
         
+        // Need to flush GL commands 
+        // to see any result on the pbuffer's front buffer.
+        GL11.glFlush();
+
         int frameRate = settings.getFrameRate();
         if (frameRate >= 1) {
             Display.sync(frameRate);