demos: move staging texture to demo struct
demos: move staging texture to demo struct

Change-Id: I8b0319d0f0bab8aba6c41e8fa324f0a9f7128baf

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers