demos: Select alpha based on capabilities in cube
demos: Select alpha based on capabilities in cube

Change-Id: I44e3654ee8d2f1b127284ae5f66ca75cb9ff54a7

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