Implement non-XR Gamepad discovery and input (#31200)

* Create embedder event to send to constellation

* Handle gamepad message in constellation, send to script thread

* Handle GamepadEvent in script thread and dispatch event to document

* Add missing Clones, fix event

* Add gamepad task source

* Adjust GamepadIndex type, remove unused imports

* Add internal getter for gamepads list

* Update gamepad new methods

* Handle gamepad connect and disconnect events

* Proto will be none, no need for HandleObject

* Initialize buttons and axes to standard mapping

* Adjust update type index types

* Update GamepadButton update function

* Adjust Gamepad mapping comments to match spec, add update logic

* Amend comment

* Update button and axis inputs on Updated event

* Add GilRs as gamepad backend in servoshell

* Add spec links, queue gamepad updates on task source

* ./mach fmt

* Fix comment length

* Split out button init, update spec comments

* Move gamepad event handling from document to global

* Map and normalize axes/button values

* Use std::time for gamepad timestamp

* Adjust gamepad handling in event loop

* Move button press/touch check into map+normalize function

- Small change but is more in line with spec

* ./mach fmt

* Update comment spec links and warning messages

* Doc comments -> regular comments

* Add window event handlers for gamepad connect/disconnect

* Adjust gamepad disconnect behavior

* Add missing TODO's, adjust gamepad/gamepadbutton list methods and formatting

* Update button handling from gilrs, add comments

* Enable gamepad pref during WPT tests and update expectations

* Update WPT expectations in meta-legacy-layout

components/shared/compositing/constellation_msg.rs

@@ -14,8 +14,8 @@ use msg::constellation_msg::{
     BrowsingContextId, PipelineId, TopLevelBrowsingContextId, TraversalDirection,
 };
 use script_traits::{
-    AnimationTickType, CompositorEvent, LogEntry, MediaSessionActionType, WebDriverCommandMsg,
-    WindowSizeData, WindowSizeType,
+    AnimationTickType, CompositorEvent, GamepadEvent, LogEntry, MediaSessionActionType,
+    WebDriverCommandMsg, WindowSizeData, WindowSizeType,
 };
 use servo_url::ServoUrl;
 
@@ -82,6 +82,8 @@ pub enum ConstellationMsg {
     IMEDismissed,
     /// Compositing done, but external code needs to present.
     ReadyToPresent(TopLevelBrowsingContextId),
+    /// Gamepad state has changed
+    Gamepad(GamepadEvent),
 }
 
 impl fmt::Debug for ConstellationMsg {
@@ -117,6 +119,7 @@ impl fmt::Debug for ConstellationMsg {
             IMEDismissed => "IMEDismissed",
             ClearCache => "ClearCache",
             ReadyToPresent(..) => "ReadyToPresent",
+            Gamepad(..) => "Gamepad",
         };
         write!(formatter, "ConstellationMsg::{}", variant)
     }