gtk4: Use async-channel instead of the glib MainContext channel

The latter will be removed in favour of using async code in the future,
and async code generally allows for more flexible message handling than the
callback based MainContext channel.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1375>

video/gtk4/Cargo.toml

@@ -21,6 +21,7 @@ gst_gl = { package = "gstreamer-gl", git = "https://gitlab.freedesktop.org/gstre
 gst_gl_wayland = { package = "gstreamer-gl-wayland", git = "https://gitlab.freedesktop.org/gstreamer/gstreamer-rs", features = ["v1_16"], optional = true }
 gst_gl_x11 = { package = "gstreamer-gl-x11", git = "https://gitlab.freedesktop.org/gstreamer/gstreamer-rs", features = ["v1_16"], optional = true }
 gst_gl_egl = { package = "gstreamer-gl-egl", git = "https://gitlab.freedesktop.org/gstreamer/gstreamer-rs", features = ["v1_16"], optional = true }
+async-channel = "2.0.0"
 
 
 [target.'cfg(target_os = "macos")'.dependencies]

video/gtk4/src/sink/imp.rs

@@ -13,7 +13,7 @@ use super::SinkEvent;
 use crate::sink::frame::Frame;
 use crate::sink::paintable::Paintable;
 
-use glib::{thread_guard::ThreadGuard, Sender};
+use glib::thread_guard::ThreadGuard;
 use gtk::prelude::*;
 use gtk::{gdk, glib};
 
@@ -60,7 +60,7 @@ static CAT: Lazy<gst::DebugCategory> = Lazy::new(|| {
 pub struct PaintableSink {
     paintable: Mutex<Option<ThreadGuard<Paintable>>>,
     info: Mutex<Option<gst_video::VideoInfo>>,
-    sender: Mutex<Option<Sender<SinkEvent>>>,
+    sender: Mutex<Option<async_channel::Sender<SinkEvent>>>,
     pending_frame: Mutex<Option<Frame>>,
     cached_caps: Mutex<Option<gst::Caps>>,
 }
@@ -449,10 +449,16 @@ impl VideoSinkImpl for PaintableSink {
             gst::FlowError::Flushing
         })?;
 
-        sender.send(SinkEvent::FrameChanged).map_err(|_| {
-            gst::error!(CAT, imp: self, "Have main thread receiver shut down");
-            gst::FlowError::Flushing
-        })?;
+        match sender.try_send(SinkEvent::FrameChanged) {
+            Ok(_) => (),
+            Err(async_channel::TrySendError::Full(_)) => {
+                gst::warning!(CAT, imp: self, "Have too many pending frames");
+            }
+            Err(async_channel::TrySendError::Closed(_)) => {
+                gst::error!(CAT, imp: self, "Have main thread receiver shut down");
+                return Err(gst::FlowError::Flushing);
+            }
+        }
 
         Ok(gst::FlowSuccess::Ok)
     }
@@ -521,20 +527,24 @@ impl PaintableSink {
         gst::debug!(CAT, imp: self, "Initializing paintable");
 
         // The channel for the SinkEvents
-        let (sender, receiver) = glib::MainContext::channel(glib::Priority::DEFAULT);
-        let self_ = self.to_owned();
+        let (sender, receiver) = async_channel::bounded(3);
 
+        // Spawn an async task on the main context to handle the channel messages
+        let main_context = glib::MainContext::default();
+
+        let self_ = self.downgrade();
+        main_context.spawn(async move {
+            while let Ok(action) = receiver.recv().await {
+                let Some(self_) = self_.upgrade() else {
+                    break;
+                };
+
+                self_.do_action(action);
+            }
+        });
+
+        // Create the paintable from the main thread
         let paintable = utils::invoke_on_main_thread(move || {
-            // Attach the receiver from the main thread to make sure it is called
-            // from a place where it can acquire the default main context.
-            receiver.attach(
-                Some(&glib::MainContext::default()),
-                glib::clone!(
-                    @weak self_ => @default-return glib::ControlFlow::Break,
-                    move |action| self_.do_action(action)
-                ),
-            );
-
             #[cfg(any(target_os = "macos", target_os = "windows", feature = "gst_gl"))]
             {
                 let gdk_context = if let GLContext::Initialized { gdk_context, .. } =