There is no easy way to pass position-relative-035 & also
pass both tests/ref/border_rounding_1px_invisible_issue_7184 and
tests/ref/text_decoration_underline_subpx.
https://github.com/servo/servo/pull/7161, "Snap rectangles to nearest
pixels...", did fix position-relative-035 (except
for OS X) but broke 1px borders.
Snapping the top-left and bottom-right corners separately can cause a
rectangle to change size when offset by a subpixel amount. Instead, this
patch snaps the top-left corner, then rounds the size to a whole pixel amount,
so any rectangle of a given size will always have the same snapped size.
Fixes#7152.
