Snap rectangles to nearest pixels consistently.

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.

tests/wpt/metadata-css/css21_dev/html4/position-relative-035.htm.ini

@@ -1,3 +1,4 @@
 [position-relative-035.htm]
   type: reftest
-  expected: FAIL
+  expected:
+    if os == "mac": FAIL