Add new round check on MovieClip

<em>I've added a check of round being >=0 on MovieClip updateTransform. This way you'll be able to set up a negative animationSpeed to play MovieClips backwards.</em>

This is a new pull request with only the MovieClip file as @GoodBoyDigital asked for in #144 I apologize for the inconveniences, that was my first pull request ever :sweat_smile: