metacity window corners not rounded
 

I've been trying some themes and window border styles, but on my machine the borders that are supposed to be rounded are still drawn as a rectangle. The bits of the corners that are supposed to be see-through are filled with white or some other color that differs from theme to theme.
In metacity screenshots i've seen so far, borders were clean so i assume the problem lies with my system.
I'd really like my windows to look good, and i don't feel like switching to another windowmanager since metacity works very neatly with gnome.
I've search both google and these forums, but i haven't found a thing.

I'm using gentoo on an Asus notebook, with a 855GME graphics chip.
Both the gnome and metacity installations are at version 2.12.2
When i emerged metacity i noticed it said that using xorg with composite together with metacity may cause "unwanted border effects", wich sounds a bit like my problem, only i don't have composite enabled in my xorg.conf.

If you feel like helping me out a bit and you need some more specifics please ask.

i figure the problem is not with metacity, since gromit causes my screen to go black.