demos: make present mode a command line option
demos: make present mode a command line option

Change-Id: I0dedd685225f5aecaba968ba81da487e5d05da26

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