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/openvr/openvr_init.cpp

@@ -6,6 +6,7 @@
 #include <QtLogging>
 #include <QtDebug>
 #include "../openvr/ivrinput_manifest.h"
+#include <QByteArray>
 
 namespace openvr_init
 {
@@ -136,4 +137,74 @@ void initializeOpenVR( const OpenVrInitializationType initType, int count )
     }
 }
 
+// Calls an OpenVR "Get...ExtensionsRequired" function and splits its
+// space-separated result. `query( buffer, size )` returns the size needed,
+// including the terminating zero.
+template <typename Query>
+static QByteArrayList queryExtensionList( Query query )
+{
+    const uint32_t size = query( nullptr, 0 );
+    if ( size == 0 )
+    {
+        return {};
+    }
+    QByteArray buffer( static_cast<int>( size ), '\0' );
+    query( buffer.data(), size );
+    QByteArrayList result;
+    for ( const QByteArray& ext :
+          buffer.left( buffer.indexOf( '\0' ) ).split( ' ' ) )
+    {
+        if ( !ext.isEmpty() )
+        {
+            result.append( ext );
+        }
+    }
+    return result;
+}
+
+// Same as the template's VrRuntime::renderInterop(). The functions are called
+// by the Vulkan backend while it creates its instance and device.
+RenderInterop renderInterop()
+{
+    RenderInterop interop;
+    interop.vulkanInstanceExtensions = []
+    {
+        vr::IVRCompositor* compositor = vr::VRCompositor();
+        if ( !compositor )
+        {
+            qWarning() << "IVRCompositor unavailable: cannot query required "
+                          "Vulkan instance extensions";
+            return QByteArrayList();
+        }
+        return queryExtensionList(
+            [&]( char* buffer, uint32_t size )
+            {
+                return compositor->GetVulkanInstanceExtensionsRequired( buffer,
+                                                                        size );
+            } );
+    };
+    interop.vulkanDeviceExtensions = []( VkPhysicalDevice_T* device )
+    {
+        vr::IVRCompositor* compositor = vr::VRCompositor();
+        if ( !compositor )
+        {
+            return QByteArrayList();
+        }
+        return queryExtensionList(
+            [&]( char* buffer, uint32_t size )
+            {
+                return compositor->GetVulkanDeviceExtensionsRequired(
+                    device, buffer, size );
+            } );
+    };
+    interop.vulkanOutputDevice = []( VkInstance_T* instance )
+    {
+        uint64_t device = 0;
+        vr::VRSystem()->GetOutputDevice(
+            &device, vr::TextureType_Vulkan, instance );
+        return reinterpret_cast<VkPhysicalDevice_T*>( device );
+    };
+    return interop;
+}
+
 } // namespace openvr_init