From what I can tell there is no good fix for this bug, calculating the 2d/3d viewborder and then attempting to align them to be pixel perfect fails because of float imprecision.
Added a workaround, so the camera border is always drawn in 2d space,
since this workaround may cause problems later on its kept under the define VIEW3D_CAMERA_BORDER_HACK so we can get old behavior back easily.