eframe: Fix window flashing white on launch (#3631)

This is a bug that can occur on Windows, previously it was fixed by only
showing the window after the first frame was rendered, but the bug
appeared again with egui 0.24.0. This commit fixes this bug by making
the window invisible on startup.

## Before


![window-flash](https://github.com/emilk/egui/assets/108888572/31a675af-8c30-46b1-a1af-653491cf67da)

## After


![no-window-flash](https://github.com/emilk/egui/assets/108888572/e7bfc429-033e-4842-8591-b9229143a60b)

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Closes https://github.com/emilk/egui/issues/3625.

crates/eframe/src/native/glow_integration.rs

@@ -154,7 +154,8 @@ impl GlowWinitApp {
             event_loop,
             native_options,
             window_settings,
-        );
+        )
+        .with_visible(false); // Start hidden until we render the first frame to fix white flash on startup (https://github.com/emilk/egui/pull/3631)
 
         let mut glutin_window_context = unsafe {
             GlutinWindowContext::new(egui_ctx, winit_window_builder, native_options, event_loop)?

crates/eframe/src/native/wgpu_integration.rs

@@ -837,7 +837,9 @@ fn create_window(
         event_loop,
         native_options,
         window_settings,
-    );
+    )
+    .with_visible(false); // Start hidden until we render the first frame to fix white flash on startup (https://github.com/emilk/egui/pull/3631)
+
     let window = {
         crate::profile_scope!("WindowBuilder::build");
         create_winit_window_builder(egui_ctx, event_loop, viewport_builder.clone())