prefs: Move some DebugOptions to Preferences and clean up (#34998)

- Move options configuring antialiasing and WebRender shader precache to
  the `Preferences` to group them with other related WebRender and DOM
  settings.
- Remove the option to disable antialiasing for canvases. This was
  unused.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

resources/wpt-prefs.json

@@ -1,4 +1,5 @@
 {
   "dom_webxr_test": true,
+  "gfx_text_antialiasing_enabled": false,
   "network_tls_ignore_unexpected_eof": true
 }