This may be retina display specific.
When using AddRect(), I do not get squared corners on rectangles. Instead, the vertical and horizontal lines intersect at the vertex and leave the corner pixel unfilled. You can see this behavior in the custom render demo by turning off the antialiasing (works as expected when antialias turned on). The squares in the demo will have unfilled corners.
I have easily gotten around with my own helper that just draws the four lines with the end points adjusted by half a pixel so the lines properly form square corners.
Is this expected, or is this an issue?