demos: Fix cube ActualTimeLate function

demos: Fix cube ActualTimeLate function

The ActualTimeLate function in cube.c is supposed to check if a
VkPastPresentationTimingGOOGLE structure indicates that the
presentation happened within a refresh-cycle duration of the
desired presentation time.

However currently ActualTimeLate will always return false, because
it is calculating the presentation deadline as `actual + rdur`.
This change calculated the presentation deadline as
`desired + rdur`.

Reviewed-by: Andy Ritger <aritger@nvidia.com>