This patch adds a placeholder for the vulkan backend.
When activated (WITH_VULKAN_BACKEND=On and --gpu-backend vulkan)
it might open a blender screen, but nothing should be visible as
none of the functions are implemented or otherwise crash on a nullptr.

This is expected as this is just a placeholder. The goal is to add shader compilation
+validation to this backend as one of the next steps so we can validate
changes to existing shaders on OpenGL, Metal and Vulkan at the same time.