This also allows us to remove the path generation and interpolation
handling for `rect()` and `xywh()` since that occurs after computation
Regressions in clip-path-interpolation-xywh.html are due to improper
simplification of length-percentage mixes where the length is 0px.
Fixes a crash on https://www.lego.com/en-gb/product/game-boy-72046 :^)
The apparent regressions in clip-path-interpolation-xywh-rect.html are
because of false positives. Something about the test was causing it to
compare two wrong values that happened to be the same. Now one of the
values is correct, they don't match.