Shader visibility is currently ignored, but we don't want to create Vulkan descriptor sets for CPU heaps.
-- v2: vkd3d: Do not create Vulkan descriptor sets for non-shader-visible heaps. vkd3d: Return a null handle from GetGPUDescriptorHandleForHeapStart() for non-shader-visible heaps. tests: Test GetGPUDescriptorHandleForHeapStart() on a non-shader-visible heap. vkd3d: Enable Vulkan-backed heaps for each heap instead of per device.