With the introduction of metal and vulkan we use a different
GPU backend selection which broke the dialog box for unsupported
platforms.
Blender asserted and segfaulted before the dialog was being shown
to the user.
This patch solves this by introducing a dummy GPU backend in case no
GPU backend was supported by OpenGL, Metal and Vulkan Backend.
It also adds the showMessageBox to GHOST_SystemCocoa.
Related to #110335
Pull Request: https://projects.blender.org/blender/blender/pulls/110919
dd4ab50065