simple question, clearing jit.window

Hello.
I have a very simple patch to run 4 videos of differing resolutions.
In order to deal with the various resolutions and aspect ratios, I'm resizing and repositioning my jit.window when each video is loaded into jit.qt.movie.
So, for example, when I load a 640X480 movie, I set a 1024X768 window at 0,0.
When I load a 1024X576 movie (720P scaled down to fit 1024), I set a 1024X576 window at 0,103 so that it's centered on the screen and not squished as it would be if I went full screen. My desktop background is black, so it basically just looks letterboxed.
I did it this way because I needed the patch to work tomorrow night, and I'm too new to jitter (though not to max) to do anything gl related to make this work.
Everything is working just fine and dandy. I just have one little quirk I'd love to be able to fix.
When I resize and reposition the window, sometimes portions of the window are white until I start playing the next video. I think I understand why this is happening, but I'd like to be able to fix it if possible.
Basically, I just want to be able to dump a black matrix to the window object to essentially clear it out.
Unfortunately, I don't quite understand the matrix stuff enough yet to make it work. My guess is that it's very simple. Can this be done?
Here's my existing patch.
(FYI my position coordinates are actually 1366,0 because I'm positioning the videos on a secondary monitor and my laptop's display is 1366X768).
Thanks.