api: throw an error if an application use a video element in multiple instance of the RxPlayer

[Florent-Bouisset]

The RxPlayer was not designed to run multiple instance of RxPlayer with the same videoElement. Having multiple instance sharing the same video element would result in various issues and unexpected behavior.

This commit adds a check to throw an error if a videoElement is shared between RxPlayer instances. This will provide better guidance to application developers regarding any errors in their implementation.