Added 'playReverse' method, and extracted part of play method (issue phaserjs#3837)

Author: khaleb85

src/gameobjects/components/Animation.js

@@ -496,6 +496,50 @@ var Animation = new Class({
             return this.parent;
         }
 
+        this.forward = true;
+        return this._startAnimation(key, startFrame);
+    },
+
+    /**
+     * Plays an Animation (in reverse mode) on the Game Object that owns this Animation Component.
+     *
+     * @method Phaser.GameObjects.Components.Animation#playReverse
+     * @fires Phaser.GameObjects.Components.Animation#onStartEvent
+     *
+     * @param {string} key - The string-based key of the animation to play, as defined previously in the Animation Manager.
+     * @param {boolean} [ignoreIfPlaying=false] - If an animation is already playing then ignore this call.
+     * @param {integer} [startFrame=0] - Optionally start the animation playing from this frame index.
+     *
+     * @return {Phaser.GameObjects.GameObject} The Game Object that owns this Animation Component.
+     */
+    playReverse: function (key, ignoreIfPlaying, startFrame)
+    {
+        if (ignoreIfPlaying === undefined) { ignoreIfPlaying = false; }
+        if (startFrame === undefined) { startFrame = 0; }
+
+        if (ignoreIfPlaying && this.isPlaying && this.currentAnim.key === key)
+        {
+            return this.parent;
+        }
+
+        this.forward = false;
+        return this._startAnimation(key, startFrame);
+    },
+
+    /**
+     * Load an Animation and fires 'onStartEvent' event,
+     * extracted from 'play' method
+     *
+     * @method Phaser.GameObjects.Components.Animation#_startAnimation
+     * @fires Phaser.GameObjects.Components.Animation#onStartEvent
+     *
+     * @param {string} key - The string-based key of the animation to play, as defined previously in the Animation Manager.
+     * @param {integer} [startFrame=0] - Optionally start the animation playing from this frame index.
+     *
+     * @return {Phaser.GameObjects.GameObject} The Game Object that owns this Animation Component.
+     */
+    _startAnimation: function (key, startFrame)
+    {
         this.load(key, startFrame);
 
         var anim = this.currentAnim;
@@ -505,8 +549,7 @@ var Animation = new Class({
         this.repeatCounter = (this._repeat === -1) ? Number.MAX_VALUE : this._repeat;
 
         anim.getFirstTick(this);
-
-        this.forward = true;
+        
         this.isPlaying = true;
         this.pendingRepeat = false;