demos: Move staging texture to demo struct
demos: Move staging texture to demo struct

Get rid of flush in staging code where it's probably unexpected, considering
it doesn't begin another command buffer, and then wait until the expected
flush to free staging texture resources

Change-Id: I80945c3f002a7efe212608895b91ccf0934fe6f9

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