Auto merge of #7161 - mbrubeck:underline, r=pcwalton

Snap rectangles to nearest pixels consistently.

Snapping the top-left and bottom-right corners separately can cause a rectangle to change size or even become empty when offset by a subpixel amount.  Instead, this patch snaps the top-left corner, then snaps the size to a whole pixel amount, so any rectangle of a given original size will always have the same snapped size.

Fixes #7152. r? @pcwalton or @glennw

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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