Fix issue #1839 (Memory Leak in DefaultLightFilter)

Co-authored-by: Lukas Habring <lukas@Lukas-PC>

jme3-core/src/main/java/com/jme3/light/DefaultLightFilter.java

@@ -59,6 +59,7 @@ public final class DefaultLightFilter implements LightFilter {
         for (Light light : processedLights) {
             light.frustumCheckNeeded = true;
         }
+        processedLights.clear();
     }
 
     @Override