Sorry, I misunderstood or just didn't read the post properly.
This will work for you, but you need to be creative.
If you haven't already got it, download the free PAINT.NET.
Open paint and create a new file as big as you can to allow for motion in PD.
Now paint the whole background the colour you would like to appear around your mask. Now select LAYER and add a second layer. On this second layer draw the shape of a mask using, say the ellipse tool. When you have the shape of a mask on layer 2 and you are happy with it, select the background layer. Now select the eraser and use it to remove the background from your mask. You should now have a coloured background and the mask should be clear.
Now go to IMAGE and select FLATTEN. Now you can save the file to your preferred folder.
Back in PD have your movie clip in track 1 and bring your new mask file into track 2. Stretch the mask file to the length of your video clip. Double click on the mask file and do what you did before. This time the video will be static and the mask will move as you dictated.
Forgive me if I have forgotten something, but it should be OK
Panny TM10, GH2, GH4,