As fas as I can tell, my Tiger desktop screen is perfectly rectangular when I hide the menu bar; so it’s probably the menu bar that has rounded corners. And since the menu is in principal always visible on a Mac, you get rounded corners (only the top left & right though).
I don’t know exactly what Displaperture does, but the easiest way to mimic this is to trim your pc background picture with rounded corners (perhaps Displaperture overlays the desktop picture with black rounded corners?). This won’t show the corners if you put a window over it, but then again, I’d suspect that’s the same for Displaperture (or for that matter, with Tiger, but that’s normally not possible to check, since you can’t drag an application over the menu bar).