rhi: Fix intro doc snippet with Vulkan

The QVulkanInstance must outlive the QRhi (if Vulkan is used).
Otherwise subtle problems may pop up upon application exit.

Pick-to: 6.6
Change-Id: Ia7074c7f53633d51cf3bbdcc84e7f578214d9648
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/doc/snippets/rhioffscreen/main.cpp

@@ -11,6 +11,9 @@ int main(int argc, char **argv)
 {
     QGuiApplication app(argc, argv);
 
+#if QT_CONFIG(vulkan)
+    QVulkanInstance inst;
+#endif
     std::unique_ptr<QRhi> rhi;
 #if defined(Q_OS_WIN)
     QRhiD3D12InitParams params;
@@ -19,7 +22,6 @@ int main(int argc, char **argv)
     QRhiMetalInitParams params;
     rhi.reset(QRhi::create(QRhi::Metal, &params));
 #elif QT_CONFIG(vulkan)
-    QVulkanInstance inst;
     inst.setExtensions(QRhiVulkanInitParams::preferredInstanceExtensions());
     if (inst.create()) {
         QRhiVulkanInitParams params;