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/net/image_cache.rs

@@ -17,6 +17,7 @@ use net_traits::image_cache::{
 use net_traits::request::CorsSettings;
 use net_traits::{FetchMetadata, FetchResponseMsg, FilteredMetadata, NetworkError};
 use pixels::{load_from_memory, CorsStatus, Image, ImageMetadata, PixelFormat};
+use servo_config::pref;
 use servo_url::{ImmutableOrigin, ServoUrl};
 use webrender_api::units::DeviceIntSize;
 use webrender_api::{ImageDescriptor, ImageDescriptorFlags, ImageFormat};
@@ -422,8 +423,8 @@ impl ImageCache for ImageCacheImpl {
         // If no information can be obtained about the system, uses 4 threads as a default
         let thread_count = thread::available_parallelism()
             .map(|i| i.get())
-            .unwrap_or(servo_config::pref!(threadpools.fallback_worker_num) as usize)
-            .min(servo_config::pref!(threadpools.async_runtime_workers.max).max(1) as usize);
+            .unwrap_or(pref!(threadpools_fallback_worker_num) as usize)
+            .min(pref!(threadpools_async_runtime_workers_max).max(1) as usize);
 
         ImageCacheImpl {
             store: Arc::new(Mutex::new(ImageCacheStore {