Keeping an "aspect ratio" on a widget

[nico <nbd free fr>]

Hello
I try to set an application who display video, and I wan't if the main
window is resized, to preserve an aspect ratio on the Drawing area
widget (within a viewport)  who display the video.

I need to fix this aspect ratio as 720/576 (width/height).

I did not find a way to do that.
Somebody have an idea on how managing this feature ?

Thanks in advance

Nico