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/constellation/pipeline.rs

@@ -3,7 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use std::borrow::Cow;
-use std::collections::{HashMap, HashSet};
+use std::collections::HashSet;
 use std::rc::Rc;
 use std::sync::Arc;
 
@@ -39,8 +39,7 @@ use script_traits::{
 };
 use serde::{Deserialize, Serialize};
 use servo_config::opts::{self, Opts};
-use servo_config::prefs;
-use servo_config::prefs::PrefValue;
+use servo_config::prefs::{self, Preferences};
 use servo_url::ServoUrl;
 use webrender_api::DocumentId;
 use webrender_traits::CrossProcessCompositorApi;
@@ -283,7 +282,7 @@ impl Pipeline {
                     load_data: state.load_data.clone(),
                     script_port,
                     opts: (*opts::get()).clone(),
-                    prefs: prefs::pref_map().iter().collect(),
+                    prefs: Box::new(prefs::get().clone()),
                     pipeline_namespace_id: state.pipeline_namespace_id,
                     webrender_document: state.webrender_document,
                     cross_process_compositor_api: state
@@ -300,7 +299,7 @@ impl Pipeline {
                 // Spawn the child process.
                 //
                 // Yes, that's all there is to it!
-                let bhm_control_chan = if opts::multiprocess() {
+                let bhm_control_chan = if opts::get().multiprocess {
                     let (bhm_control_chan, bhm_control_port) =
                         ipc::channel().expect("Sampler chan");
                     unprivileged_pipeline_content.bhm_control_port = Some(bhm_control_port);
@@ -490,7 +489,7 @@ pub struct UnprivilegedPipelineContent {
     load_data: LoadData,
     script_port: IpcReceiver<ConstellationControlMsg>,
     opts: Opts,
-    prefs: HashMap<String, PrefValue>,
+    prefs: Box<Preferences>,
     pipeline_namespace_id: PipelineNamespaceId,
     cross_process_compositor_api: CrossProcessCompositorApi,
     webrender_document: DocumentId,
@@ -581,7 +580,7 @@ impl UnprivilegedPipelineContent {
         self.opts.clone()
     }
 
-    pub fn prefs(&self) -> HashMap<String, PrefValue> {
-        self.prefs.clone()
+    pub fn prefs(&self) -> &Preferences {
+        &self.prefs
     }
 }