vulkan: Make gsk_renderer_realize() work with NULL surface

vulkan: Make gsk_renderer_realize() work with NULL surface

Pretty much copy what GL does and just use the default display to create
GPU-related resources without the need for a display.

This also adds gdk_display_create_vulkan_context() but I've
kept it private because the Vulkan API is generally considered in flux,
in particular with our pending attempts to redo how renderers work.