fix: emit particles at initial Entity zIndex instead of zIndex 0, closes

 #1318

fxgl-entity/src/main/kotlin/com/almasb/fxgl/particle/ParticleComponent.kt

@@ -36,6 +36,7 @@ open class ParticleComponent(val emitter: ParticleEmitter) : Component() {
 
     override fun onUpdate(tpf: Double) {
         if (parent.world == null) {
+            parent.zIndex = entity.zIndex
             entity.world.addEntity(parent)
         }
 

fxgl-entity/src/test/kotlin/com/almasb/fxgl/particle/ParticleComponentTest.kt

@@ -0,0 +1,56 @@
+/*
+ * FXGL - JavaFX Game Library. The MIT License (MIT).
+ * Copyright (c) AlmasB (almaslvl@gmail.com).
+ * See LICENSE for details.
+ */
+@file:Suppress("JAVA_MODULE_DOES_NOT_DEPEND_ON_MODULE")
+package com.almasb.fxgl.particle
+
+import com.almasb.fxgl.entity.Entity
+import com.almasb.fxgl.entity.GameWorld
+import com.almasb.fxgl.entity.component.ComponentHelper
+import org.hamcrest.CoreMatchers.`is`
+import org.hamcrest.MatcherAssert.assertThat
+import org.junit.jupiter.api.Assertions.assertNotNull
+import org.junit.jupiter.api.Assertions.assertNull
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+
+/**
+ *
+ * @author Jean-Rene Lavoie (jeanrlavoie@gmail.com)
+ */
+class ParticleComponentTest {
+
+    private lateinit var world: GameWorld
+    private lateinit var particle: ParticleComponent
+
+    @BeforeEach
+    fun setUp() {
+        world = GameWorld()
+        particle = ParticleComponent(ParticleEmitter())
+    }
+
+    @Test
+    fun `Create ParticleComponent with zIndex`() {
+        assertNull(particle.entity)
+        assertNotNull(particle.parent)
+        assertThat(particle.parent.zIndex, `is`(0))
+
+        val e = Entity()
+        e.zIndex = 100
+
+        ComponentHelper.setEntity(particle, e)
+        world.addEntity(e)
+        particle.onAdded()
+        particle.onUpdate(1.0)
+
+        assertThat(particle.parent.zIndex, `is`(100))
+
+        e.zIndex = 200
+        particle.onUpdate(1.0)
+
+        assertThat(particle.parent.zIndex, `is`(100))
+    }
+
+}