demos: Update cubepp
demos: Update cubepp

Modify correct uniform buffer for next frame.
Fix fence sync after new throttling scheme.
Select alpha based on capabilities in cube.
Add uniform buffer per swapchain image to cube.
Cube updated to use triple buffering.

Change-Id: Ia929bde0e6c3c2129dc9164a65696af1ee0c0329

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