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>

CMakeLists.txt

@@ -13,11 +13,9 @@ set(CMAKE_VS_INCLUDE_INSTALL_TO_DEFAULT_BUILD 1)
 #find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS Gui Multimedia Qml Quick WebSockets Widgets QuickControls2 QuickDialogs2 Svg)
 #find_package(Qt${QT_VERSION_MAJOR} OPTIONAL_COMPONENTS DBus)
 # Core5Compat removed: nothing uses it, and the Steam Frame's system Qt
-# doesn't ship it.
-# GuiPrivate: QRhi (overlaycontroller.cpp) needs it. Since Qt 6.10 private
-# modules must be requested explicitly. Temporary: it goes away when rendering
-# switches to public API (docs/steamframe/DESIGN.md, section 3).
-find_package(Qt6 REQUIRED COMPONENTS Gui Multimedia Qml Quick WebSockets Widgets QuickControls2 QuickDialogs2 Svg Qml GuiPrivate)
+# doesn't ship it. No private Qt modules: the binary must run on another Qt 6
+# patch release, e.g. the Frame's system Qt (docs/steamframe/DESIGN.md).
+find_package(Qt6 REQUIRED COMPONENTS Gui Multimedia Qml Quick WebSockets Widgets QuickControls2 QuickDialogs2 Svg Qml)
 find_package(Vulkan REQUIRED)
 
 qt_standard_project_setup(REQUIRES 6.8)
@@ -69,6 +67,8 @@ set(SRCS
     src/utils/paths.cpp
     src/utils/setup.cpp
     src/utils/update_rate.cpp
+    src/render/RenderBackend.cpp
+    src/render/OpenGLRenderBackend.cpp
 )
 
 set(HEADERS
@@ -112,6 +112,8 @@ set(HEADERS
     src/utils/setup.h
     src/utils/update_rate.h
     src/utils/platform_capabilities.h
+    src/render/RenderBackend.h
+    src/render/OpenGLRenderBackend.h
     src/utils/Matrix.h
     src/quaternion/quaternion.h
     )
@@ -291,6 +293,21 @@ target_compile_definitions(AdvancedSettings PRIVATE
     APPLICATION_VERSION=\"${PROJECT_VERSION}\"
 )
 
+# Vulkan render backend (src/render/VulkanRenderBackend.*), selected at run
+# time with --graphics-api vulkan. Needs a Qt built with Vulkan support; the
+# Vulkan headers come from find_package(Vulkan) above. Nothing links against
+# libvulkan: Qt loads it at run time.
+if (QT_FEATURE_vulkan)
+    target_sources(AdvancedSettings PRIVATE
+        src/render/VulkanRenderBackend.cpp
+        src/render/VulkanRenderBackend.h
+    )
+    target_compile_definitions(AdvancedSettings PRIVATE OVRAS_HAS_VULKAN=1)
+else ()
+    target_compile_definitions(AdvancedSettings PRIVATE OVRAS_HAS_VULKAN=0)
+endif ()
+message(STATUS "Vulkan render backend: ${QT_FEATURE_vulkan}")
+
 # Build for the Steam Frame (SteamOS, aarch64). Switches off what doesn't work
 # there and changes some defaults; the details are named capability constants
 # in src/utils/platform_capabilities.h, the only place reading this definition.
@@ -324,8 +341,6 @@ target_link_directories(AdvancedSettings PRIVATE ${OPENVR_LIB_DIR})
 
 target_link_libraries(AdvancedSettings PRIVATE
     Qt::Gui
-    # For QRhi; temporary, see find_package(Qt6 ...) above.
-    Qt::GuiPrivate
     Qt::Multimedia
     Qt::Qml
     Qt::Quick