Add prefs to limit threadpool sizes (#34478)

* Add prefs to limit threadpool sizes

Add preferences to control the size of threadpools,
so that we can easily reduce the amount of runtime
threads and test which pools benefit from more
threads.

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

* Add pref for Webrender threadpool

Add a preference to limit the size of the webrender threadpool.
Note: WebRender by default calls hooks which register the threads with
a profiler instance that the embedder can register with webrender.
Servo currently doesn't register such a profiler with webrender,
but in the future we might also want to profile the
webrender threadpool.

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

---------

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

components/config/prefs.rs

@@ -198,6 +198,28 @@ mod gen {
                 #[serde(rename = "fonts.default-monospace-size")]
                 default_monospace_size: i64,
             },
+            /// Allows customizing the different threadpools used by servo
+            threadpools: {
+                /// Number of workers per threadpool, if we fail to detect how much
+                /// parallelism is available at runtime.
+                fallback_worker_num: i64,
+                image_cache_workers: {
+                    /// Maximum number of workers for the Image Cache thread pool
+                    max: i64,
+                },
+                async_runtime_workers: {
+                    /// Maximum number of workers for the Networking async runtime thread pool
+                    max: i64
+                },
+                resource_workers: {
+                    /// Maximum number of workers for the Core Resource Manager
+                    max: i64,
+                },
+                webrender_workers: {
+                    /// Maximum number of workers for webrender
+                    max: i64,
+                },
+            },
             css: {
                 animations: {
                     testing: {