Implemented the prefers-reduced-transparency media query for all
platforms.
Windows and Mac have specific settings which are used, others (Android
and Linux/GTK) have it enabled if prefers-reduced-motion is also enabled
as there is no dedicated setting to check.
Locked behind new pref `layout.css.prefers-reduced-transparency.enabled`,
off by default always for now.
Also added new WPT tests (none previously).
Demo video: https://goose.icu/firefox_prt.mp4
Test page: https://goose.icu/prefers-reduced-transparency
Differential Revision: https://phabricator.services.mozilla.com/D172424
