Fix GIF memory overflow issue.

Runtime/ui/painting/codec_gif.cs

         volatile int _repetitionCount;
         volatile bool _isDone;
         volatile int _frameIndex;
+        volatile Texture2D _texture;
         readonly UIWidgetsCoroutine _coroutine;
 
         public GifCodec(byte[] bytes) {
             return frame.Then(frameData => {
                 this._nextFrame();
 
-                if (frameData.frameInfo != null) {
-                    return frameData.frameInfo;
+                if (this._texture == null) {
+                    this._texture = new Texture2D(this._width, this._height, TextureFormat.BGRA32, false);
+                    this._texture.hideFlags = HideFlags.HideAndDontSave;
-                var tex = new Texture2D(this._width, this._height, TextureFormat.BGRA32, false);
-                tex.hideFlags = HideFlags.HideAndDontSave;
+                var tex = this._texture;
+                if (frameData.frameInfo != null) {
+                    return frameData.frameInfo;
+                }
+
-                frameData.gifFrame = null; // dispose gifFrame
+                // frameData.gifFrame = null; // dispose gifFrame
 
                 return frameData.frameInfo;
             });