Add backward animation demo

[ido-ran]

You say that you add backward animation feature in V3 - can you please add demo that show how to use it?

[akdcl]

You can use AnimationState.timescale to backward animation. https://github.com/DragonBones/DragonBonesJS/blob/master/DragonBones/src/dragonBones/animation/AnimationState.ts#L110-L126