layout: Rewrite clipping to be a two-phase process that takes physical

border box positions and transforms into account.

Clipping region computation now follows a simple process: (1) in the
parent's coordinate system, parents store appropriate clipping regions
into children; (2) each child moves its clipping region to its own
coordinate system if necessary.

Because clipping region computation is now based on stacking-relative
border box positions and the `transform_rect` method, it can handle
`position: relative` offsets and more types of transforms, such as
scaling.

Improves etsy.com.

Closes #13753.

tests/wpt/metadata-css/css-transforms-1_dev/html/transform-transformable-inline-block.htm.ini

@@ -1,3 +0,0 @@
-[transform-transformable-inline-block.htm]
-  type: reftest
-  expected: FAIL