Render through public Qt API: OpenGL and Vulkan backends, --graphics-api

Replace the QRhi rendering in OverlayController (private Qt API) with the
render backends of the vrdashboard template (src/render/), which use public
Qt API only (QQuickRenderTarget::fromOpenGLTexture / fromVulkanImage). The
binary then runs on another Qt 6 patch release, e.g. the Steam Frame's system
Qt.

The Vulkan backend also does what SteamVR requires and the QRhi path lacked:
the required instance/device extensions and the HMD's GPU (via
openvr_init::renderInterop()), VK_FORMAT_R8G8B8A8_UNORM, and the transition to
TRANSFER_SRC_OPTIMAL after each frame.

- New option --graphics-api opengl|vulkan (default opengl, no fallback).
- Shutdown() calls VR_Shutdown before the render target and Qt's device go
  (crash on the Steam Frame with Vulkan otherwise); it runs only once.
- Mouse y is flipped on every platform: SteamVR reports it in GL convention.
- Render delay 5 -> 16 ms (about 60 fps).
- GuiPrivate is no longer needed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/utils/setup.cpp

@@ -58,6 +58,12 @@ CommandLineOptions returnCommandLineParser( const MyQApplication& application )
                                       k_resetSettingsDescription );
     parser.addOption( resetSettings );
 
+    QCommandLineOption graphicsApi( k_graphicsApi,
+                                    k_graphicsApiDescription,
+                                    k_graphicsApiValueName,
+                                    k_graphicsApiDefault );
+    parser.addOption( graphicsApi );
+
     parser.process( application );
 
     const bool desktopModeEnabled = parser.isSet( desktopMode );
@@ -84,11 +90,16 @@ CommandLineOptions returnCommandLineParser( const MyQApplication& application )
     if ( resetSettingsEnabled )
         qInfo() << "Reset SteamVR Settings.";
 
-    const CommandLineOptions commandLineArgs{
-        desktopModeEnabled,         forceNoSoundEnabled,
-        forceNoManifestEnabled,     forceInstallManifestEnabled,
-        forceRemoveManifestEnabled, resetSettingsEnabled
-    };
+    const QString graphicsApiValue = parser.value( graphicsApi );
+    qInfo() << "Graphics API:" << graphicsApiValue;
+
+    const CommandLineOptions commandLineArgs{ desktopModeEnabled,
+                                              forceNoSoundEnabled,
+                                              forceNoManifestEnabled,
+                                              forceInstallManifestEnabled,
+                                              forceRemoveManifestEnabled,
+                                              resetSettingsEnabled,
+                                              graphicsApiValue };
 
     qInfo() << "Command line arguments processed.";