demos: Add compositeAlphaFlags fix to cube.c
demos: Add compositeAlphaFlags fix to cube.c

Change-Id: Id19d11e909ffc21c4aa6a419b071ba53594ae94f

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