If there was a category on Mantis for the website itself it would make it possible to forego this forum and have all issues in one place.
The distinct bug tracking features would allow the reporter to be more detailed about website issues.
I am suggesting this feature, specifically hoping that it will be a foundation for making it easier to get the forum into a more streamlined state that aligns well with the other Kdenlive resources.

Using the Drupal bugtracker sounds like a good way to integrate the management of bugs into the site.
However, unless someone is willing to port all bugs from Mantis over to the Drupal bugtracker, I'd be opposed to it. Otherwise there would now be two bugtrackers and I can only image the additional problems that would create (e.g. what if the wrong one was chosen? does the admin now have to move it by hand?).
If everything can be in Drupal, great, but it sounds like a lot of work.