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/main.cpp

@@ -1,6 +1,7 @@
 #include "utils/setup.h"
 #include "settings/settings.h"
 #include "openvr/ovr_settings_wrapper.h"
+#include "render/RenderBackend.h"
 
 #include <QtLogging>
 #include <QtDebug>
@@ -46,6 +47,37 @@ int main( int argc, char* argv[] )
 
     openvr_init::initializeOpenVR(
         openvr_init::OpenVrInitializationType::Overlay );
+
+    // Render backend (render/RenderBackend.h). Created after OpenVR, because
+    // the Vulkan backend needs SteamVR's device requirements
+    // (openvr_init::renderInterop()), and before OverlayController, because
+    // QQuickWindow::setGraphicsApi() must run before any QQuickWindow exists
+    // (OverlayController owns one).
+    GraphicsApi graphicsApi = GraphicsApi::OpenGL;
+    if ( !parseGraphicsApi( commandLineArgs.graphicsApi, &graphicsApi ) )
+    {
+        qCritical() << "Unknown graphics API:" << commandLineArgs.graphicsApi
+                    << "(expected opengl or vulkan)";
+        return ReturnErrorCode::GENERAL_FAILURE;
+    }
+    std::unique_ptr<RenderBackend> renderBackend
+        = createRenderBackend( graphicsApi );
+    if ( !renderBackend )
+    {
+        qCritical() << "Graphics API" << commandLineArgs.graphicsApi
+                    << "is not compiled into this build";
+        return ReturnErrorCode::GENERAL_FAILURE;
+    }
+    QString renderBackendError;
+    if ( !renderBackend->initialize( openvr_init::renderInterop(),
+                                     &renderBackendError ) )
+    {
+        qCritical() << renderBackend->name()
+                    << "initialization failed:" << renderBackendError;
+        return ReturnErrorCode::GENERAL_FAILURE;
+    }
+    QQuickWindow::setGraphicsApi( renderBackend->graphicsApi() );
+
     try
     {
         QQmlEngine qmlEngine;
@@ -61,7 +93,8 @@ int main( int argc, char* argv[] )
         std::unique_ptr<advsettings::OverlayController> controller(
             new advsettings::OverlayController( commandLineArgs.desktopMode,
                                                 commandLineArgs.forceNoSound,
-                                                qmlEngine ) );
+                                                qmlEngine,
+                                                std::move( renderBackend ) ) );
         //const auto url
         //    = QUrl( ":/qt/qml/AdvancedSettings/src/res/qml/common/mainwidget.qml" );
         QQmlComponent component( &qmlEngine, ":/qt/qml/AdvancedSettings/src/res/qml/common/mainwidget.qml" );