QT: Hide GLWidget immediately after showing.

With the loading screen merged, we don't want to actually show at this
point, but it still needs to be shown to actually create the context.
Turns out you can just show and hide it immediately and it'll work.

src/yuzu/bootmanager.cpp

@@ -393,6 +393,8 @@ void GRenderWindow::InitRenderTarget() {
     BackupGeometry();
     // show causes the window to actually be created and the gl context as well
     show();
+    // but we don't want the widget to be shown yet, so immediately hide it
+    hide();
 }
 
 void GRenderWindow::CaptureScreenshot(u16 res_scale, const QString& screenshot_path) {