Image: request mipmaps to be re-generated if image data is modified

- fixes ImageRaster modification not updating mipmaps bug

jme3-core/src/main/java/com/jme3/texture/Image.java

@@ -427,6 +427,18 @@ public class Image extends NativeObject implements Savable /*, Cloneable*/ {
         return needGeneratedMips;
     }
     
+    /**
+     * Sets the update needed flag, while also checking if mipmaps
+     * need to be regenerated.
+     */
+    @Override
+    public void setUpdateNeeded() {
+        super.setUpdateNeeded();
+        if (!isGeneratedMipmapsRequired() && !hasMipmaps()) {
+            setNeedGeneratedMipmaps();
+        }
+    }
+    
     /**
      * Determine if the image is NPOT.
      *