Use Abortable future for StagingBelt recall with GLib spawning

In the original code, it was canceled by dropping the local executor.
We need to do the same here to make destruction work fine.

wgpu/src/window/compositor.rs

@@ -1,6 +1,7 @@
 use crate::{Backend, Color, Error, Renderer, Settings, Viewport};
 
-use futures::task::SpawnExt;
+use futures::future::{AbortHandle, Abortable};
+use futures::prelude::*;
 use iced_native::{futures, mouse};
 use raw_window_handle::HasRawWindowHandle;
 
@@ -12,6 +13,7 @@ pub struct Compositor {
     device: wgpu::Device,
     queue: wgpu::Queue,
     staging_belt: wgpu::util::StagingBelt,
+    belt_emergency_stop: Option<AbortHandle>,
 }
 
 impl Compositor {
@@ -60,6 +62,7 @@ impl Compositor {
             device,
             queue,
             staging_belt,
+            belt_emergency_stop: None,
         })
     }
 
@@ -168,8 +171,21 @@ impl iced_graphics::window::Compositor for Compositor {
         self.queue.submit(Some(encoder.finish()));
 
         // Recall staging buffers
-        glib::MainContext::default().spawn(self.staging_belt.recall());
+        let (abort_handle, abort_registration) = AbortHandle::new_pair();
+        self.belt_emergency_stop = Some(abort_handle);
+        glib::MainContext::default().spawn(
+            Abortable::new(self.staging_belt.recall(), abort_registration)
+                .map(|_| ()),
+        );
 
         mouse_interaction
     }
 }
+
+impl Drop for Compositor {
+    fn drop(&mut self) {
+        if let Some(abort_handle) = self.belt_emergency_stop.take() {
+            abort_handle.abort();
+        }
+    }
+}