cube: Fill in pPlatformWindow after you have a window
cube: Fill in pPlatformWindow after you have a window

demo_init_vk(), which sets the surface_description.pPlatformWindow was
getting called before the window was created so surface_description.pPlatformWindow
was always NULL. I added a new demo_init_vk_wsi() which splits demo_init_vk()
in two so that the window is created before surface_description.pPlatformWindow is assigned.

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools