api: Flatten and simplify Servo preferences (#34966)

Flatten and simplify Servo's preferences code. In addition, have both
preferences and options passed in as arguments to `Servo::new()` and
make sure not to use the globally set preferences in `servoshell` (as
much as possible now).

Instead of a complex procedural macro to generate preferences, just
expose a very simple derive macro that adds string based getters and
setters.

- All command-line parsing is moved to servoshell.
- There is no longer the concept of a missing preference.
- Preferences no longer have to be part of the resources bundle because
  they now have reasonable default values.
- servoshell specific preferences are no longer part of the preferences
  exposed by the Servo API.

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

components/compositing/compositor.rs

@@ -2303,7 +2303,7 @@ impl<Window: WindowMethods + ?Sized> IOCompositor<Window> {
         self.assert_gl_framebuffer_complete();
 
         // Set the viewport background based on prefs.
-        let color = servo_config::pref!(shell.background_color.rgba);
+        let color = servo_config::pref!(shell_background_color_rgba);
         gl.clear_color(
             color[0] as f32,
             color[1] as f32,