Hoow to get rid of the dark grey square surrounding video in the jit.pwindow

Hello.
I sure this is a newbie question.
I'm a beginning with Jitter and openGL. and trying to get used to jit.world instead of using jit.gl.render.
So here's my question :
I made a patch in which I want to superimpose 4 videos in the same jit.pwindow.
I will be using the jit.world's jit.gl.node function for this purpose.
In the example patch I have join, there is only one video playing for now.
What I trying to achieve is ; to get rid of the dark grey square surrounding the video in the jit.pwindow.
Does it have to do with the videoplane of the subcontext on which the video reside or does it have to do with jit.gl.node?
I can't figure it out.
Thanks a lot for your help.