fix #313 #312 onBlendModeChanged _paint LateInitializationError

[CarGuo]

[VERBOSE-2:ui_dart_state.cc(209)] Unhandled Exception: Exception: LateInitializationError: Field '_paint@268459774' has not been initialized.
#0      FlutterActorImage._paint (package:flare_flutter/flare.dart)
#1      FlutterActorImage.onBlendModeChanged (package:flare_flutter/flare.dart:505:5)
#2      FlutterActorDrawable.blendMode= (package:flare_flutter/flare.dart:226:5)
#3      FlutterActorDrawable.blendModeId= (package:flare_flutter/flare.dart:234:5)
#4      ActorDrawable.read (package:flare_flutter/base/actor_drawable.dart:77:17)
#5      ActorImage.read (package:flare_flutter/base/actor_image.dart:305:19)
#6      ActorArtboard.readComponentsBlock (package:flare_flutter/base/actor_artboard.dart:387:34)
#7      ActorArtboard.read (package:flare_flutter/base/actor_artboard.dart:349:11)

In onBlendModeChanged Function, _paint never initialized when ActorDrawable read -> set blendModeId ，but _paint sound late ui.Paint

  @override
  void onBlendModeChanged(ui.BlendMode mode) {
     _paint.blendMode = mode;
    onPaintUpdated(_paint);
  }

[amorenew]

@luigi-rosso Could you merge it?
It solves the unhandled exception issue
Yes, Rive is there but Developer Quest uses the advanced functions in Flare