Remove flash between poster and video

[cmmartin]

This resolves #1128

[n1ru4l]

Could you please explain why this prevents the flashing?

[cmmartin]

If you put a breakpoint in the dealloc method of RTCVideo.m, it will be fired once the video loads and the poster is removed. So, the current implementation allocates a native video player, renders it behind a poster, then once the video loads, it de-allocates the original instance and creates a new instance. That re-instantiation causes the flash. After applying my changes, dealloc is not called when the video loads and the flash is gone.

I don't know enough about the internals of React to say for sure why it is not re-using the native video player upon video load, but I would guess the fact that the RCTVideo component changes parents.

In other words, the switch from...

 <View style={nativeProps.style}>	
        <RCTVideo	
            ref={this._assignRoot}	
            {...nativeProps}	
        />
       ...
</View>

to simply...

<RCTVideo	
   ref={this._assignRoot}	
   {...nativeProps}	
/>

leads to a creation of a new RCTVideo instance

[n1ru4l]

@cobarx Lets merge this!

[cobarx]

@n1ru4l Ok, feel free to approve any of these open PRs and I'll merge them. I've been short on time to do the extensive testing to make sure that many of these work.

Before approve anything, please review and make sure the code is readable, test it, and make sure any new functionality is documented.