Auto merge of #23758 - jdm:compositing-canvas-break, r=gterzian

Remove canvas->compositing dependency

There's one tiny place in the canvas crate that depends on the compositing crate, and it's really quite backwards. If we use trait objects instead it becomes much cleaner to reason about.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because no tests for windows-only webgl code.

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components/servo/lib.rs

@@ -64,7 +64,7 @@ fn webdriver(_port: u16, _constellation: Sender<ConstellationMsg>) {}
 
 use bluetooth::BluetoothThreadFactory;
 use bluetooth_traits::BluetoothRequest;
-use canvas::gl_context::GLContextFactory;
+use canvas::gl_context::{CloneableDispatcher, GLContextFactory};
 use canvas::webgl_thread::WebGLThreads;
 use compositing::compositor_thread::{
     CompositorProxy, CompositorReceiver, InitialCompositorState, Msg,
@@ -100,6 +100,7 @@ use media::{GLPlayerThreads, WindowGLContext};
 use msg::constellation_msg::{PipelineNamespace, PipelineNamespaceId};
 use net::resource_thread::new_resource_threads;
 use net_traits::IpcSend;
+use offscreen_gl_context::GLContextDispatcher;
 use profile::mem as profile_mem;
 use profile::time as profile_time;
 use profile_traits::mem;
@@ -745,7 +746,8 @@ fn create_constellation(
     let gl_factory = if opts.should_use_osmesa() {
         GLContextFactory::current_osmesa_handle()
     } else {
-        GLContextFactory::current_native_handle(&compositor_proxy)
+        let dispatcher = Box::new(MainThreadDispatcher::new(compositor_proxy.clone())) as Box<_>;
+        GLContextFactory::current_native_handle(dispatcher)
     };
 
     let (external_image_handlers, external_images) = WebrenderExternalImageHandlers::new();
@@ -937,3 +939,29 @@ fn create_sandbox() {
 fn create_sandbox() {
     panic!("Sandboxing is not supported on Windows, iOS, ARM targets and android.");
 }
+
+/// Implements GLContextDispatcher to dispatch functions from GLContext threads to the main thread's event loop.
+/// It's used in Windows to allow WGL GLContext sharing.
+pub struct MainThreadDispatcher {
+    compositor_proxy: CompositorProxy,
+}
+
+impl MainThreadDispatcher {
+    fn new(proxy: CompositorProxy) -> Self {
+        Self {
+            compositor_proxy: proxy,
+        }
+    }
+}
+impl GLContextDispatcher for MainThreadDispatcher {
+    fn dispatch(&self, f: Box<dyn Fn() + Send>) {
+        self.compositor_proxy.send(Msg::Dispatch(f));
+    }
+}
+impl CloneableDispatcher for MainThreadDispatcher {
+    fn clone(&self) -> Box<dyn GLContextDispatcher> {
+        Box::new(MainThreadDispatcher {
+            compositor_proxy: self.compositor_proxy.clone(),
+        }) as Box<_>
+    }
+}