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/shared/canvas/webgl.rs

@@ -33,7 +33,7 @@ pub fn webgl_channel<T>() -> Option<(WebGLSender<T>, WebGLReceiver<T>)>
 where
     T: for<'de> Deserialize<'de> + Serialize,
 {
-    base::generic_channel::channel(servo_config::opts::multiprocess())
+    base::generic_channel::channel(servo_config::opts::get().multiprocess)
 }
 
 /// Entry point channel type used for sending WebGLMsg messages to the WebGL renderer.