Part 2: Add at <position> into ray() in style system.

We reuse PositionOrAuto here, and let "auto" represent the situation when the
author omits "at <position>" because it has a special meaning.

https://drafts.fxtf.org/motion-1/#valdef-ray-at-position

Note: No need to update css/motion/parsing/offset-path-parsing-valid.html
because Blink added some to the upstream repo already.

Differential Revision: https://phabricator.services.mozilla.com/D179860

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1820070
gecko-commit: c9a0635b6d98eaac89582e4bfff31d6e330dc4e6
gecko-reviewers: emilio

css/motion/parsing/offset-path-computed.html

@@ -21,6 +21,11 @@
 test_computed_value("offset-path", "ray(270deg farthest-corner contain)");
 test_computed_value("offset-path", "ray(-720deg sides)");
 test_computed_value("offset-path", "ray(calc(180deg - 45deg) farthest-side)", "ray(135deg farthest-side)");
+test_computed_value("offset-path", "ray(0deg at 100px 100px)");
+test_computed_value("offset-path", "ray(0deg sides at center center)", "ray(0deg sides at 50% 50%)");
+test_computed_value("offset-path", "ray(0deg at center center sides)", "ray(0deg sides at 50% 50%)");
+test_computed_value("offset-path", "ray(0deg at center center contain)", "ray(0deg contain at 50% 50%)");
+test_computed_value("offset-path", "ray(at 10px 10px 0deg contain)", "ray(0deg contain at 10px 10px)");
 
 // It's unclear about the normalization at computed time, so we accept both
 // cases for now.