Created attachment 18307 [details]
Testcase (no self-checking, though)
I'll attach a testcase (c) and the output images I get on my Mac. Zooming into the images will show off-by-0.5 drawing errors.
As Behdad mentioned on IRC »you should get sharp lines«
Created attachment 18308 [details]
Created attachment 18309 [details]
Just to save this information; vlad told me that this bug seems to be fixed on Mac OS X 10.5, so it's my 10.4 setup that appears to expose this bug.
This seems to be an old Quartz (not cairo-quartz) bug, that has been fixed in more recent MacOS X versions.