Use RAII for `GHOST_WindowClass`.
For the main Blender executable, only one `GHOST_System` is ever
created. But when running tests, this can happen multiple times (eg,
GPUOpenGLTest and GPUOpenGLWorkaroundsTest).
This results in `GHOST_SystemWin32` trying to register
`GHOST_WindowClass` multiple times, so `RegisterClassW` fails with
`ERROR_CLASS_ALREADY_EXISTS`.
Pull Request: https://projects.blender.org/blender/blender/pulls/144231
7169a39b79