servo: Merge #13840 - WebGL support on Windows (from MortimerGoro:webrender_dispatcher); r=emilio

<!-- Please describe your changes on the following line: -->
This is the final step to provide WebGL support on Windows ;)

Some Related PRs already merged in webrender and offscreen-gl-context:

https://github.com/emilio/rust-offscreen-rendering-context/pull/64
https://github.com/servo/webrender/pull/432

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

Source-Repo: https://github.com/servo/servo
Source-Revision: 4216224f9cc7f3430db2b59f4d77061824ba7a1e

servo/components/compositing/compositor.rs

@@ -354,6 +354,17 @@ impl webrender_traits::RenderNotifier for RenderNotifier {
     }
 }
 
+// Used to dispatch functions from webrender to the main thread's event loop.
+struct CompositorThreadDispatcher {
+    compositor_proxy: Box<CompositorProxy>
+}
+
+impl webrender_traits::RenderDispatcher for CompositorThreadDispatcher {
+    fn dispatch(&self, f: Box<Fn() + Send>) {
+        self.compositor_proxy.send(Msg::Dispatch(f));
+    }
+}
+
 impl<Window: WindowMethods> IOCompositor<Window> {
     fn new(window: Rc<Window>, state: InitialCompositorState)
            -> IOCompositor<Window> {
@@ -426,6 +437,15 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                                                   compositor.constellation_chan.clone());
         compositor.webrender.set_render_notifier(Box::new(render_notifier));
 
+        if cfg!(target_os = "windows") {
+            // Used to dispatch functions from webrender to the main thread's event loop.
+            // Required to allow WGL GLContext sharing in Windows.
+            let dispatcher = Box::new(CompositorThreadDispatcher {
+                compositor_proxy: compositor.channel_to_self.clone_compositor_proxy()
+            });
+            compositor.webrender.set_main_thread_dispatcher(dispatcher);
+        }
+
         // Set the size of the root layer.
         compositor.update_zoom_transform();
 
@@ -642,6 +662,12 @@ impl<Window: WindowMethods> IOCompositor<Window> {
                 }
             }
 
+            (Msg::Dispatch(func), ShutdownState::NotShuttingDown) => {
+                // The functions sent here right now are really dumb, so they can't panic.
+                // But if we start running more complex code here, we should really catch panic here.
+                func();
+            }
+
             // When we are shutting_down, we need to avoid performing operations
             // such as Paint that may crash because we have begun tearing down
             // the rest of our resources.

servo/components/compositing/compositor_thread.rs

@@ -126,6 +126,10 @@ pub enum Msg {
     // sends a reply on the IpcSender, the constellation knows it's safe to
     // tear down the other threads associated with this pipeline.
     PipelineExited(PipelineId, IpcSender<()>),
+    /// Runs a closure in the compositor thread.
+    /// It's used to dispatch functions from webrender to the main thread's event loop.
+    /// Required to allow WGL GLContext sharing in Windows.
+    Dispatch(Box<Fn() + Send>)
 }
 
 impl Debug for Msg {
@@ -158,6 +162,7 @@ impl Debug for Msg {
             Msg::PipelineVisibilityChanged(..) => write!(f, "PipelineVisibilityChanged"),
             Msg::PipelineExited(..) => write!(f, "PipelineExited"),
             Msg::NewScrollFrameReady(..) => write!(f, "NewScrollFrameReady"),
+            Msg::Dispatch(..) => write!(f, "Dispatch"),
         }
     }
 }