cube: demonstrate how to select other VkPresentModeKHR's

cube: demonstrate how to select other VkPresentModeKHR's

This comes from the Khronos-internal discussion of
https://github.com/LunarG/VulkanSamples/issues/98.  Cube will, by default,
choose VK_PRESENT_FIFO_KHR mode.  However, when that change was made, the old
(example) code was lost that showed how to search for the other present modes.
It was also discussed that there isn't good guidance on why an application
might want to use a mode other than FIFO.  This change adds both of those, but
only selects FIFO by default.  #ifdef's can be used to select other modes.
With that code is a reason why an application might wish to use that present
mode.