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/render/OpenGLRenderBackend.h

@@ -0,0 +1,57 @@
+// Adapted from the vrdashboard template (FrameAssistant commit 4a5b090),
+// verified on the Steam Frame. See docs/steamframe/DESIGN.md, section 3.
+
+#pragma once
+
+#include <QOffscreenSurface>
+#include <QOpenGLContext>
+
+#include "RenderBackend.h"
+
+class OpenGLRenderBackend final : public RenderBackend
+{
+public:
+    OpenGLRenderBackend() = default;
+    ~OpenGLRenderBackend() override;
+
+    QString name() const override
+    {
+        return QStringLiteral( "OpenGL" );
+    }
+    QSGRendererInterface::GraphicsApi graphicsApi() const override
+    {
+        return QSGRendererInterface::OpenGL;
+    }
+
+    bool initialize( const RenderInterop& interop, QString* error ) override;
+    void configureWindow( QQuickWindow* window ) override;
+    bool attachWindow( QQuickWindow* window, QString* error ) override;
+    void detachWindow() override
+    {
+        releaseRenderTarget();
+    }
+    QString deviceName() const override
+    {
+        return m_deviceName;
+    }
+
+    bool makeCurrent() override;
+    void doneCurrent() override;
+
+    QQuickRenderTarget createRenderTarget( QSize size ) override;
+    void releaseRenderTarget() override;
+    void beginFrame() override {}
+    void finishFrame() override;
+    void waitForGpu() override;
+
+    vr::Texture_t vrTexture() override;
+    vr::VRTextureBounds_t vrTextureBounds() const override;
+    QImage readback() override;
+
+private:
+    std::unique_ptr<QOpenGLContext> m_context;
+    std::unique_ptr<QOffscreenSurface> m_surface;
+    unsigned int m_texture = 0; // GLuint
+    QSize m_size;
+    QString m_deviceName;
+};