Disable OpenXR runtime on macOS and remove stub module

[jcfr]

Since attempting to build on macOS results into few errors described below, this commit sets the default value for SlicerVirtualReality_HAS_OPENXR_SUPPORT to OFF.

It also removes the VirtualRealityStub module and re-enable support for building on macOS. This means an extension will be built but it would not have any working XR runtimes.

macOS errors related to OpenXR-SDK and vtkRenderingOpenXR

Issue finding "OpenGL.framework"

During the configuration of the external project associated with the
vtkRenderingVR project, error like the following was reported:

  CMake Error in /D/S/A/VTK/Rendering/VR/CMakeLists.txt:
    Imported target "VTK::RenderingOpenGL2" includes non-existent path
      "/Applications/Xcode-14.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.0.sdk/System/Library/Frameworks/OpenGL.framework"
    in its INTERFACE_INCLUDE_DIRECTORIES.  Possible reasons include:
    * The path was deleted, renamed, or moved to another location.
    * An install or uninstall procedure did not complete successfully.
    * The installation package was faulty and references files it does not
    provide.

It was fixed by adding the missing symlink:

cd /Applications/Xcode-14.2.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs
ln -s MacOSX.sdk MacOSX14.0.sdk

Issue building "OpenXR-SDK" project

The following was reported when linking the libopenxr_loader library:

  [  6%] Linking CXX shared library libopenxr_loader.dylib
  -macosx_version_min has been renamed to -macos_version_min
  ld: unknown options: --version-script=/D/S/S-0-E-b/SlicerVirtualReality-build/OpenXR-SDK/src/loader/openxr-loader.map --no-undefined
  clang-14: error: linker command failed with exit code 1 (use -v to see invocation)

Updating "OpenXR-SDK" from release-1.0.26 to release-1.0.32 allows to
address this.

Issue resolving "XrGraphicsBindingOpenGL"

Here is the error reported while building the vtkRenderingOpenXR module:

    [ 15%] Building CXX object vtkRenderingOpenXR/CMakeFiles/RenderingOpenXR.dir/vtkOpenXRManagerConnection.cxx.o
  In file included from /D/S/A/VTK/Rendering/OpenXR/vtkOpenXRManagerOpenGLGraphics.cxx:15:
  /D/S/A/VTK/Rendering/OpenXR/vtkOpenXRManagerOpenGLGraphics.h:119:19: error: use of undeclared identifier 'XrGraphicsBindingOpenGL'
    std::shared_ptr<XrGraphicsBindingOpenGL> GraphicsBinding;
                    ^
  /D/S/A/VTK/Rendering/OpenXR/vtkOpenXRManagerOpenGLGraphics.h:49:16: error: cannot initialize return object of type 'vtkObjectBase *' with an rvalue of type 'vtkOpenXRManagerOpenGLGraphics *'
    vtkTypeMacro(vtkOpenXRManagerOpenGLGraphics, vtkOpenXRManagerGraphics);
    ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[jcfr]

With this change, here the configuration output. Since it was done from an existing build tree, the variable habe been explicitly removed to simulate a clean build.

cmake \
  -USlicerVirtualReality_HAS_OPENVR_SUPPORT \
  -USlicerVirtualReality_HAS_OPENXR_SUPPORT .

[...]
-- 
-- Setting SlicerVirtualReality_HAS_OPENVR_SUPPORT to OFF (OpenVR not supported on macOS)
-- 
-- 
-- Setting SlicerVirtualReality_HAS_OPENXR_SUPPORT to OFF (OpenXR not supported on macOS)
-- 
-- SlicerVirtualReality_EXTERNAL_PROJECT_DEPENDENCIES:vtkRenderingVR
-- Remote - VTKExternalModule [OK]
-- Remote - VTKExternalModule_SOURCE_DIR:/D/S/S-0-E-b/SlicerVirtualReality-build/VTKExternalModule
-- SuperBuild - First pass
-- SuperBuild - First pass - done
-- SuperBuild - inner => Requires vtkRenderingVR, 
-- SuperBuild -   vtkRenderingVR[OK]
-- SuperBuild -   VTK_SOURCE_DIR:/Users/svc-dashboard/D/S/A/VTK-build/../VTK
-- SuperBuild -   vtkRenderingVR_DIR:/D/S/S-0-E-b/SlicerVirtualReality-build/vtkRenderingVR-build
-- SuperBuild - inner[OK]
-- Configuring done
-- Generating done
-- Build files have been written to: /D/S/S-0-E-b/SlicerVirtualReality-build

We can observe the two XR runtime OpenVR and OpenXR are disabled. Only the generic base vtkRenderingVR module is being built.

...
[100%] Built target qSlicerVirtualRealityModuleGenericCxxTests
[ 94%] Forcing configure step for 'inner'
[ 94%] No install step for 'inner'
[100%] Completed 'inner'
[100%] Built target inner