How to play video behind mask in Impress?

Hello all,

For a project I will be projecting my presentation on a circular projection screen.
To avoid light beams are projected besides the projection screen I created a mask in Impress. Behind this mask I put the content I would like to show. The mask is always the object on the foreground of each slide.

I created a mask which basically is a black rectangle shape with a circular open area shape in the middle. I created this mask using the subtract function of these 2 shapes.

This technique works fine for images and text, but when playing video’s during presentation these video’s are played full screen and ignore the mask they are put behind.

Anybody knows why this is happening and any solution you can recommend?
I know this technique works in Powerpoint and Keynote.

Many thanks for your help and ideas.