Fix black flash on start in glow eframe backend (#7616)

Before:


https://github.com/user-attachments/assets/b31b7a4f-a5f7-45af-bfe0-4c8e174e209f

After:


https://github.com/user-attachments/assets/47544987-ca81-4efb-b778-5ca59b7fc0ac

I think to make things even nicer we could also try calling
request_discard when a window is shown

crates/eframe/src/native/glow_integration.rs

@@ -563,6 +563,12 @@ impl GlowWinitRunning<'_> {
             (raw_input, viewport_ui_cb)
         };
 
+        // HACK: In order to get the right clear_color, the system theme needs to be set, which
+        // usually only happens in the `update` call. So we call Options::begin_pass early
+        // to set the right theme. Without this there would be a black flash on the first frame.
+        self.integration
+            .egui_ctx
+            .options_mut(|opt| opt.begin_pass(&raw_input));
         let clear_color = self
             .app
             .clear_color(&self.integration.egui_ctx.style().visuals);