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>

components/shared/canvas/lib.rs

@@ -19,7 +19,6 @@ pub enum ConstellationCanvasMsg {
     Create {
         id_sender: Sender<CanvasId>,
         size: Size2D<u64>,
-        antialias: bool,
     },
     Exit,
 }