Wrapping overlays in GtkGraphicsOffload causes composition problems; add
them directly to GtkOverlay so they use the normal GTK rendering path.

diff --git a/src/app/window/mod.rs b/src/app/window/mod.rs
--- a/src/app/window/mod.rs
+++ b/src/app/window/mod.rs
@@ -31,7 +31,7 @@
     pub fn set_overlay(&self, widget: &impl IsA<Widget>) {
         let window = self.imp();
 
-        window.overlay.add_overlay(&graphics_offload(widget));
+        window.overlay.add_overlay(widget);
     }
 
     pub fn set_fullscreen(&self, fullscreen: bool) {
@@ -65,11 +65,3 @@
         self.imp().open_file(file_path);
     }
 }
-
-fn graphics_offload(widget: &impl IsA<Widget>) -> gtk::GraphicsOffload {
-    gtk::GraphicsOffload::builder()
-        .vexpand(true)
-        .hexpand(true)
-        .child(widget)
-        .build()
-}