[Impeller] Vulkan backend needs to support platforms that only support combined depth-stencil.

See https://github.com/flutter/engine/blob/main/impeller/renderer/backend/vulkan/capabilities_vk.cc#L351

We will lose ~25% of all Vulkan capable devices by requiring stencil only formats.