Add MacOS support

[jcfr]

If anybody would use MacOS for virtual reality and you already have virtual reality capable hardware then post a comment here. Also describe how can you contribute to the effort.

[lassoan]

TODO: Fix packaging of OpenVR framework

Since OpenVR.framework is a directory, it should be installed as such.

https://github.com/KitwareMedical/SlicerOpenVR/blob/17c78d31edd43fc686c2f38fed4a0212323c79ec/SuperBuild/External_OpenVR.cmake#L52-L54

The following code should be improved:

https://github.com/KitwareMedical/SlicerOpenVR/blob/17c78d31edd43fc686c2f38fed4a0212323c79ec/CMakeLists.txt#L48-L53

[jcfr]

Support for building the extension was made possible through the following pull requests:

• https://github.com/KitwareMedical/SlicerVirtualReality/pull/160
• https://github.com/KitwareMedical/SlicerVirtualReality/pull/161
• https://github.com/KitwareMedical/SlicerVirtualReality/pull/150
• https://github.com/KitwareMedical/SlicerVirtualReality/pull/163 (to support building OpenXR-SDK on macOS)
• https://github.com/KitwareMedical/SlicerVirtualReality/pull/164

While the extension build, it doesn't provide any integration with XR environment. Enabling OpenXR leads to the following failure when building vtkRenderingOpenXR:

Issue related to "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]

Closing. We should revisit the macOS support once VTK itself support it.

[lassoan]

We need to keep this issue open so that people who are waiting for macOS support can keep monitoring this issue. Maybe we could add labels etc. to distinguish this feature request from issues.