gtk4: Flush frames from the paintable when shutting down the sink

Otherwise it will continue showing the last frames forever and keep
around the frames forever instead of rendering black.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/issues/281

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1024>

video/gtk4/src/sink/imp.rs

@@ -221,6 +221,14 @@ impl ElementImpl for PaintableSink {
             gst::StateChange::PausedToReady => {
                 let _ = self.info.lock().unwrap().take();
                 let _ = self.pending_frame.lock().unwrap().take();
+
+                let self_ = self.to_owned();
+                utils::invoke_on_main_thread(move || {
+                    let paintable = self_.paintable.lock().unwrap();
+                    if let Some(paintable) = &*paintable {
+                        paintable.get().handle_flush_frames();
+                    }
+                });
             }
             _ => (),
         }

video/gtk4/src/sink/paintable/imp.rs

@@ -196,4 +196,12 @@ impl Paintable {
             self.obj().invalidate_contents();
         }
     }
+
+    pub(super) fn handle_flush_frames(&self) {
+        gst::debug!(CAT, imp: self, "Flushing frames");
+        self.paintables.borrow_mut().clear();
+        self.cached_textures.borrow_mut().clear();
+        self.obj().invalidate_size();
+        self.obj().invalidate_contents();
+    }
 }

video/gtk4/src/sink/paintable/mod.rs

@@ -33,4 +33,8 @@ impl Paintable {
     pub(crate) fn handle_frame_changed(&self, frame: Option<Frame>) {
         self.imp().handle_frame_changed(frame);
     }
+
+    pub(crate) fn handle_flush_frames(&self) {
+        self.imp().handle_flush_frames();
+    }
 }